re

  This module contains regular expression matching functions for
  strings and binaries.

  The regular expression syntax and semantics resemble that of
  Perl.

  The matching algorithms of the library are based on the PCRE
  library, but not all of the PCRE library is interfaced and some
  parts of the library go beyond what PCRE offers. Currently PCRE
  version 8.40 (release date 2017-01-11) is used. The sections of
  the PCRE documentation that are relevant to this module are
  included here.

  Note:
    The Erlang literal syntax for strings uses the "\" (backslash)
    character as an escape code. You need to escape backslashes in
    literal strings, both in your code and in the shell, with an
    extra backslash, that is, "\\".

Perl-Like Regular Expression Syntax

  The following sections contain reference material for the regular
  expressions used by this module. The information is based on the
  PCRE documentation, with changes where this module behaves
  differently to the PCRE library.

PCRE Regular Expression Details

  The syntax and semantics of the regular expressions supported by
  PCRE are described in detail in the following sections. Perl's
  regular expressions are described in its own documentation, and
  regular expressions in general are covered in many books, some
  with copious examples. Jeffrey Friedl's "Mastering Regular
  Expressions", published by O'Reilly, covers regular expressions in
  great detail. This description of the PCRE regular expressions is
  intended as reference material.

  The reference material is divided into the following sections:

   • Special Start-of-Pattern Items

   • Characters and Metacharacters

   • Backslash

   • Circumflex and Dollar

   • Full Stop (Period, Dot) and \N

   • Matching a Single Data Unit

   • Square Brackets and Character Classes

   • Posix Character Classes

   • Vertical Bar

   • Internal Option Setting

   • Subpatterns

   • Duplicate Subpattern Numbers

   • Named Subpatterns

   • Repetition

   • Atomic Grouping and Possessive Quantifiers

   • Back References

   • Assertions

   • Conditional Subpatterns

   • Comments

   • Recursive Patterns

   • Subpatterns as Subroutines

   • Oniguruma Subroutine Syntax

   • Backtracking Control

Special Start-of-Pattern Items

  Some options that can be passed to compile/2 can also be set by
  special items at the start of a pattern. These are not
  Perl-compatible, but are provided to make these options accessible
  to pattern writers who are not able to change the program that
  processes the pattern. Any number of these items can appear, but
  they must all be together right at the start of the pattern
  string, and the letters must be in upper case.

  UTF Support

  Unicode support is basically UTF-8 based. To use Unicode
  characters, you either call compile/2 or run/3 with option 
  unicode, or the pattern must start with one of these special
  sequences:

    (*UTF8)
    (*UTF)

  Both options give the same effect, the input string is interpreted
  as UTF-8. Notice that with these instructions, the automatic
  conversion of lists to UTF-8 is not performed by the re
  functions. Therefore, using these sequences is not recommended.
  Add option unicode when running compile/2 instead.

  Some applications that allow their users to supply patterns can
  wish to restrict them to non-UTF data for security reasons. If
  option never_utf is set at compile time, (*UTF), and so on, are
  not allowed, and their appearance causes an error.

  Unicode Property Support

  The following is another special sequence that can appear at the
  start of a pattern:

    (*UCP)

  This has the same effect as setting option ucp: it causes
  sequences such as \d and \w to use Unicode properties to determine
  character types, instead of recognizing only characters with codes <
  256 through a lookup table.

  Disabling Startup Optimizations

  If a pattern starts with (*NO_START_OPT), it has the same effect
  as setting option no_start_optimize at compile time.

  Newline Conventions

  PCRE supports five conventions for indicating line breaks in
  strings: a single CR (carriage return) character, a single LF
  (line feed) character, the two-character sequence CRLF, any of the
  three preceding, and any Unicode newline sequence.

  A newline convention can also be specified by starting a pattern
  string with one of the following five sequences:

  (*CR):
    Carriage return

  (*LF):
    Line feed

  (*CRLF):
    >Carriage return followed by line feed

  (*ANYCRLF):
    Any of the three above

  (*ANY):
    All Unicode newline sequences

  These override the default and the options specified to compile/2.
  For example, the following pattern changes the convention to CR:

    (*CR)a.b

  This pattern matches a\nb, as LF is no longer a newline. If more
  than one of them is present, the last one is used.

  The newline convention affects where the circumflex and dollar
  assertions are true. It also affects the interpretation of the dot
  metacharacter when dotall is not set, and the behavior of \N.
  However, it does not affect what the \R escape sequence matches.
  By default, this is any Unicode newline sequence, for Perl
  compatibility. However, this can be changed; see the description
  of \R in section Newline Sequences. A change of the \R setting
  can be combined with a change of the newline convention.

  Setting Match and Recursion Limits

  The caller of run/3 can set a limit on the number of times the
  internal match() function is called and on the maximum depth of
  recursive calls. These facilities are provided to catch runaway
  matches that are provoked by patterns with huge matching trees (a
  typical example is a pattern with nested unlimited repeats) and to
  avoid running out of system stack by too much recursion. When one
  of these limits is reached, pcre_exec() gives an error return.
  The limits can also be set by items at the start of the pattern of
  the following forms:

    (*LIMIT_MATCH=d)
    (*LIMIT_RECURSION=d)

  Here d is any number of decimal digits. However, the value of the
  setting must be less than the value set by the caller of run/3
  for it to have any effect. That is, the pattern writer can lower
  the limit set by the programmer, but not raise it. If there is
  more than one setting of one of these limits, the lower value is
  used.

  The default value for both the limits is 10,000,000 in the Erlang
  VM. Notice that the recursion limit does not affect the stack
  depth of the VM, as PCRE for Erlang is compiled in such a way that
  the match function never does recursion on the C stack.

  Note that LIMIT_MATCH and LIMIT_RECURSION can only reduce the
  value of the limits set by the caller, not increase them.

Characters and Metacharacters

  A regular expression is a pattern that is matched against a
  subject string from left to right. Most characters stand for
  themselves in a pattern and match the corresponding characters in
  the subject. As a trivial example, the following pattern matches a
  portion of a subject string that is identical to itself:

    The quick brown fox

  When caseless matching is specified (option caseless), letters
  are matched independently of case.

  The power of regular expressions comes from the ability to include
  alternatives and repetitions in the pattern. These are encoded in
  the pattern by the use of metacharacters, which do not stand for
  themselves but instead are interpreted in some special way.

  Two sets of metacharacters exist: those that are recognized
  anywhere in the pattern except within square brackets, and those
  that are recognized within square brackets. Outside square
  brackets, the metacharacters are as follows:

  \:
    General escape character with many uses

  ^:
    Assert start of string (or line, in multiline mode)

  $:
    Assert end of string (or line, in multiline mode)

  .:
    Match any character except newline (by default)

  [:
    Start character class definition

  |:
    Start of alternative branch

  (:
    Start subpattern

  ):
    End subpattern

  ?:
    Extends the meaning of (, also 0 or 1 quantifier, also
    quantifier minimizer

  *:
    0 or more quantifiers

  +:
    1 or more quantifier, also "possessive quantifier"

  {:
    Start min/max quantifier

  Part of a pattern within square brackets is called a "character
  class". The following are the only metacharacters in a character
  class:

  \:
    General escape character

  ^:
    Negate the class, but only if the first character

  -:
    Indicates character range

  [:
    Posix character class (only if followed by Posix syntax)

  ]:
    Terminates the character class

  The following sections describe the use of each metacharacter.

Backslash

  The backslash character has many uses. First, if it is followed by
  a character that is not a number or a letter, it takes away any
  special meaning that a character can have. This use of backslash
  as an escape character applies both inside and outside character
  classes.

  For example, if you want to match a * character, you write \* in
  the pattern. This escaping action applies if the following
  character would otherwise be interpreted as a metacharacter, so it
  is always safe to precede a non-alphanumeric with backslash to
  specify that it stands for itself. In particular, if you want to
  match a backslash, write \\.

  In unicode mode, only ASCII numbers and letters have any special
  meaning after a backslash. All other characters (in particular,
  those whose code points are > 127) are treated as literals.

  If a pattern is compiled with option extended, whitespace in the
  pattern (other than in a character class) and characters between a #
  outside a character class and the next newline are ignored. An
  escaping backslash can be used to include a whitespace or #
  character as part of the pattern.

  To remove the special meaning from a sequence of characters, put
  them between \Q and \E. This is different from Perl in that $ and @
  are handled as literals in \Q...\E sequences in PCRE, while $ and @
  cause variable interpolation in Perl. Notice the following
  examples:

    Pattern            PCRE matches   Perl matches
    
    \Qabc$xyz\E        abc$xyz        abc followed by the contents of $xyz
    \Qabc\$xyz\E       abc\$xyz       abc\$xyz
    \Qabc\E\$\Qxyz\E   abc$xyz        abc$xyz

  The \Q...\E sequence is recognized both inside and outside
  character classes. An isolated \E that is not preceded by \Q is
  ignored. If \Q is not followed by \E later in the pattern, the
  literal interpretation continues to the end of the pattern (that
  is, \E is assumed at the end). If the isolated \Q is inside a
  character class, this causes an error, as the character class is
  not terminated.

  Non-Printing Characters

  A second use of backslash provides a way of encoding non-printing
  characters in patterns in a visible manner. There is no
  restriction on the appearance of non-printing characters, apart
  from the binary zero that terminates a pattern. When a pattern is
  prepared by text editing, it is often easier to use one of the
  following escape sequences than the binary character it
  represents:

  \a:
    Alarm, that is, the BEL character (hex 07)

  \cx:
    "Control-x", where x is any ASCII character

  \e:
    Escape (hex 1B)

  \f:
    Form feed (hex 0C)

  \n:
    Line feed (hex 0A)

  \r:
    Carriage return (hex 0D)

  \t:
    Tab (hex 09)

  \0dd:
    Character with octal code 0dd

  \ddd:
    Character with octal code ddd, or back reference

  \o{ddd..}:
    character with octal code ddd..

  \xhh:
    Character with hex code hh

  \x{hhh..}:
    Character with hex code hhh..

  Note:
    Note that \0dd is always an octal code, and that \8 and \9 are
    the literal characters "8" and "9".

  The precise effect of \cx on ASCII characters is as follows: if x
  is a lowercase letter, it is converted to upper case. Then bit 6
  of the character (hex 40) is inverted. Thus \cA to \cZ become hex
  01 to hex 1A (A is 41, Z is 5A), but \c{ becomes hex 3B ({ is 7B),
  and \c; becomes hex 7B (; is 3B). If the data item (byte or 16-bit
  value) following \c has a value > 127, a compile-time error
  occurs. This locks out non-ASCII characters in all modes.

  The \c facility was designed for use with ASCII characters, but
  with the extension to Unicode it is even less useful than it once
  was.

  After \0 up to two further octal digits are read. If there are
  fewer than two digits, just those that are present are used. Thus
  the sequence \0\x\015 specifies two binary zeros followed by a CR
  character (code value 13). Make sure you supply two digits after
  the initial zero if the pattern character that follows is itself
  an octal digit.

  The escape \o must be followed by a sequence of octal digits,
  enclosed in braces. An error occurs if this is not the case. This
  escape is a recent addition to Perl; it provides way of specifying
  character code points as octal numbers greater than 0777, and it
  also allows octal numbers and back references to be unambiguously
  specified.

  For greater clarity and unambiguity, it is best to avoid following \
  by a digit greater than zero. Instead, use \o{} or \x{} to specify
  character numbers, and \g{} to specify back references. The
  following paragraphs describe the old, ambiguous syntax.

  The handling of a backslash followed by a digit other than 0 is
  complicated, and Perl has changed in recent releases, causing PCRE
  also to change. Outside a character class, PCRE reads the digit
  and any following digits as a decimal number. If the number is <
  8, or if there have been at least that many previous capturing
  left parentheses in the expression, the entire sequence is taken
  as a back reference. A description of how this works is provided
  later, following the discussion of parenthesized subpatterns.

  Inside a character class, or if the decimal number following \ is >
  7 and there have not been that many capturing subpatterns, PCRE
  handles \8 and \9 as the literal characters "8" and "9", and
  otherwise re-reads up to three octal digits following the
  backslash, and using them to generate a data character. Any
  subsequent digits stand for themselves. For example:

  \040:
    Another way of writing an ASCII space

  \40:
    The same, provided there are < 40 previous capturing
    subpatterns

  \7:
    Always a back reference

  \11:
    Can be a back reference, or another way of writing a tab

  \011:
    Always a tab

  \0113:
    A tab followed by character "3"

  \113:
    Can be a back reference, otherwise the character with octal
    code 113

  \377:
    Can be a back reference, otherwise value 255 (decimal)

  \81:
    Either a back reference, or the two characters "8" and "1"

  Notice that octal values >= 100 that are specified using this
  syntax must not be introduced by a leading zero, as no more than
  three octal digits are ever read.

  By default, after \x that is not followed by {, from zero to two
  hexadecimal digits are read (letters can be in upper or lower
  case). Any number of hexadecimal digits may appear between \x{ and
  }. If a character other than a hexadecimal digit appears between
  \x{ and }, or if there is no terminating }, an error occurs.

  Characters whose value is less than 256 can be defined by either
  of the two syntaxes for \x. There is no difference in the way they
  are handled. For example, \xdc is exactly the same as \x{dc}.

  Constraints on character values

  Characters that are specified using octal or hexadecimal numbers
  are limited to certain values, as follows:

  8-bit non-UTF mode:
    < 0x100

  8-bit UTF-8 mode:
    < 0x10ffff and a valid codepoint

  Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the
  so-called "surrogate" codepoints), and 0xffef.

  Escape sequences in character classes

  All the sequences that define a single character value can be used
  both inside and outside character classes. Also, inside a
  character class, \b is interpreted as the backspace character (hex
  08).

  \N is not allowed in a character class. \B, \R, and \X are not
  special inside a character class. Like other unrecognized escape
  sequences, they are treated as the literal characters "B", "R",
  and "X". Outside a character class, these sequences have different
  meanings.

  Unsupported Escape Sequences

  In Perl, the sequences \l, \L, \u, and \U are recognized by its
  string handler and used to modify the case of following
  characters. PCRE does not support these escape sequences.

  Absolute and Relative Back References

  The sequence \g followed by an unsigned or a negative number,
  optionally enclosed in braces, is an absolute or relative back
  reference. A named back reference can be coded as \g{name}. Back
  references are discussed later, following the discussion of
  parenthesized subpatterns.

  Absolute and Relative Subroutine Calls

  For compatibility with Oniguruma, the non-Perl syntax \g followed
  by a name or a number enclosed either in angle brackets or single
  quotes, is alternative syntax for referencing a subpattern as a
  "subroutine". Details are discussed later. Notice that \g{...}
  (Perl syntax) and \g<...> (Oniguruma syntax) are not synonymous.
  The former is a back reference and the latter is a subroutine
  call.

  Generic Character Types

  Another use of backslash is for specifying generic character
  types:

  \d:
    Any decimal digit

  \D:
    Any character that is not a decimal digit

  \h:
    Any horizontal whitespace character

  \H:
    Any character that is not a horizontal whitespace character

  \s:
    Any whitespace character

  \S:
    Any character that is not a whitespace character

  \v:
    Any vertical whitespace character

  \V:
    Any character that is not a vertical whitespace character

  \w:
    Any "word" character

  \W:
    Any "non-word" character

  There is also the single sequence \N, which matches a non-newline
  character. This is the same as the "." metacharacter when dotall
  is not set. Perl also uses \N to match characters by name, but
  PCRE does not support this.

  Each pair of lowercase and uppercase escape sequences partitions
  the complete set of characters into two disjoint sets. Any given
  character matches one, and only one, of each pair. The sequences
  can appear both inside and outside character classes. They each
  match one character of the appropriate type. If the current
  matching point is at the end of the subject string, all fail, as
  there is no character to match.

  For compatibility with Perl, \s did not used to match the VT
  character (code 11), which made it different from the the POSIX
  "space" class. However, Perl added VT at release 5.18, and PCRE
  followed suit at release 8.34. The default \s characters are now
  HT (9), LF (10), VT (11), FF (12), CR (13), and space (32), which
  are defined as white space in the "C" locale. This list may vary
  if locale-specific matching is taking place. For example, in some
  locales the "non-breaking space" character (\xA0) is recognized as
  white space, and in others the VT character is not.

  A "word" character is an underscore or any character that is a
  letter or a digit. By default, the definition of letters and
  digits is controlled by the PCRE low-valued character tables, in
  Erlang's case (and without option unicode), the ISO Latin-1
  character set.

  By default, in unicode mode, characters with values > 255, that
  is, all characters outside the ISO Latin-1 character set, never
  match \d, \s, or \w, and always match \D, \S, and \W. These
  sequences retain their original meanings from before UTF support
  was available, mainly for efficiency reasons. However, if option 
  ucp is set, the behavior is changed so that Unicode properties
  are used to determine character types, as follows:

  \d:
    Any character that \p{Nd} matches (decimal digit)

  \s:
    Any character that \p{Z} or \h or \v

  \w:
    Any character that matches \p{L} or \p{N} matches, plus
    underscore

  The uppercase escapes match the inverse sets of characters. Notice
  that \d matches only decimal digits, while \w matches any Unicode
  digit, any Unicode letter, and underscore. Notice also that ucp
  affects \b and \B, as they are defined in terms of \w and \W.
  Matching these sequences is noticeably slower when ucp is set.

  The sequences \h, \H, \v, and \V are features that were added to
  Perl in release 5.10. In contrast to the other sequences, which
  match only ASCII characters by default, these always match certain
  high-valued code points, regardless if ucp is set.

  The following are the horizontal space characters:

  U+0009:
    Horizontal tab (HT)

  U+0020:
    Space

  U+00A0:
    Non-break space

  U+1680:
    Ogham space mark

  U+180E:
    Mongolian vowel separator

  U+2000:
    En quad

  U+2001:
    Em quad

  U+2002:
    En space

  U+2003:
    Em space

  U+2004:
    Three-per-em space

  U+2005:
    Four-per-em space

  U+2006:
    Six-per-em space

  U+2007:
    Figure space

  U+2008:
    Punctuation space

  U+2009:
    Thin space

  U+200A:
    Hair space

  U+202F:
    Narrow no-break space

  U+205F:
    Medium mathematical space

  U+3000:
    Ideographic space

  The following are the vertical space characters:

  U+000A:
    Line feed (LF)

  U+000B:
    Vertical tab (VT)

  U+000C:
    Form feed (FF)

  U+000D:
    Carriage return (CR)

  U+0085:
    Next line (NEL)

  U+2028:
    Line separator

  U+2029:
    Paragraph separator

  In 8-bit, non-UTF-8 mode, only the characters with code points <
  256 are relevant.

  Newline Sequences

  Outside a character class, by default, the escape sequence \R
  matches any Unicode newline sequence. In non-UTF-8 mode, \R is
  equivalent to the following:

    (?>\r\n|\n|\x0b|\f|\r|\x85)

  This is an example of an "atomic group", details are provided
  below.

  This particular group matches either the two-character sequence CR
  followed by LF, or one of the single characters LF (line feed,
  U+000A), VT (vertical tab, U+000B), FF (form feed, U+000C), CR
  (carriage return, U+000D), or NEL (next line, U+0085). The
  two-character sequence is treated as a single unit that cannot be
  split.

  In Unicode mode, two more characters whose code points are > 255
  are added: LS (line separator, U+2028) and PS (paragraph
  separator, U+2029). Unicode character property support is not
  needed for these characters to be recognized.

  \R can be restricted to match only CR, LF, or CRLF (instead of the
  complete set of Unicode line endings) by setting option 
  bsr_anycrlf either at compile time or when the pattern is
  matched. (BSR is an acronym for "backslash R".) This can be made
  the default when PCRE is built; if so, the other behavior can be
  requested through option bsr_unicode. These settings can also be
  specified by starting a pattern string with one of the following
  sequences:

  (*BSR_ANYCRLF):
    CR, LF, or CRLF only

  (*BSR_UNICODE):
    Any Unicode newline sequence

  These override the default and the options specified to the
  compiling function, but they can themselves be overridden by
  options specified to a matching function. Notice that these
  special settings, which are not Perl-compatible, are recognized
  only at the very start of a pattern, and that they must be in
  upper case. If more than one of them is present, the last one is
  used. They can be combined with a change of newline convention;
  for example, a pattern can start with:

    (*ANY)(*BSR_ANYCRLF)

  They can also be combined with the (*UTF8), (*UTF), or (*UCP)
  special sequences. Inside a character class, \R is treated as an
  unrecognized escape sequence, and so matches the letter "R" by
  default.

  Unicode Character Properties

  Three more escape sequences that match characters with specific
  properties are available. When in 8-bit non-UTF-8 mode, these
  sequences are limited to testing characters whose code points are <
  256, but they do work in this mode. The following are the extra
  escape sequences:

  \p{xx}:
    A character with property xx

  \P{xx}:
    A character without property xx

  \X:
    A Unicode extended grapheme cluster

  The property names represented by xx above are limited to the
  Unicode script names, the general category properties, "Any",
  which matches any character (including newline), and some special
  PCRE properties (described in the next section). Other Perl
  properties, such as "InMusicalSymbols", are currently not
  supported by PCRE. Notice that \P{Any} does not match any
  characters and always causes a match failure.

  Sets of Unicode characters are defined as belonging to certain
  scripts. A character from one of these sets can be matched using a
  script name, for example:

    \p{Greek} \P{Han}

  Those that are not part of an identified script are lumped
  together as "Common". The following is the current list of
  scripts:

   • Arabic

   • Armenian

   • Avestan

   • Balinese

   • Bamum

   • Bassa_Vah

   • Batak

   • Bengali

   • Bopomofo

   • Braille

   • Buginese

   • Buhid

   • Canadian_Aboriginal

   • Carian

   • Caucasian_Albanian

   • Chakma

   • Cham

   • Cherokee

   • Common

   • Coptic

   • Cuneiform

   • Cypriot

   • Cyrillic

   • Deseret

   • Devanagari

   • Duployan

   • Egyptian_Hieroglyphs

   • Elbasan

   • Ethiopic

   • Georgian

   • Glagolitic

   • Gothic

   • Grantha

   • Greek

   • Gujarati

   • Gurmukhi

   • Han

   • Hangul

   • Hanunoo

   • Hebrew

   • Hiragana

   • Imperial_Aramaic

   • Inherited

   • Inscriptional_Pahlavi

   • Inscriptional_Parthian

   • Javanese

   • Kaithi

   • Kannada

   • Katakana

   • Kayah_Li

   • Kharoshthi

   • Khmer

   • Khojki

   • Khudawadi

   • Lao

   • Latin

   • Lepcha

   • Limbu

   • Linear_A

   • Linear_B

   • Lisu

   • Lycian

   • Lydian

   • Mahajani

   • Malayalam

   • Mandaic

   • Manichaean

   • Meetei_Mayek

   • Mende_Kikakui

   • Meroitic_Cursive

   • Meroitic_Hieroglyphs

   • Miao

   • Modi

   • Mongolian

   • Mro

   • Myanmar

   • Nabataean

   • New_Tai_Lue

   • Nko

   • Ogham

   • Ol_Chiki

   • Old_Italic

   • Old_North_Arabian

   • Old_Permic

   • Old_Persian

   • Oriya

   • Old_South_Arabian

   • Old_Turkic

   • Osmanya

   • Pahawh_Hmong

   • Palmyrene

   • Pau_Cin_Hau

   • Phags_Pa

   • Phoenician

   • Psalter_Pahlavi

   • Rejang

   • Runic

   • Samaritan

   • Saurashtra

   • Sharada

   • Shavian

   • Siddham

   • Sinhala

   • Sora_Sompeng

   • Sundanese

   • Syloti_Nagri

   • Syriac

   • Tagalog

   • Tagbanwa

   • Tai_Le

   • Tai_Tham

   • Tai_Viet

   • Takri

   • Tamil

   • Telugu

   • Thaana

   • Thai

   • Tibetan

   • Tifinagh

   • Tirhuta

   • Ugaritic

   • Vai

   • Warang_Citi

   • Yi

  Each character has exactly one Unicode general category property,
  specified by a two-letter acronym. For compatibility with Perl,
  negation can be specified by including a circumflex between the
  opening brace and the property name. For example, \p{^Lu} is the
  same as \P{Lu}.

  If only one letter is specified with \p or \P, it includes all the
  general category properties that start with that letter. In this
  case, in the absence of negation, the curly brackets in the escape
  sequence are optional. The following two examples have the same
  effect:

    \p{L}
    \pL

  The following general category property codes are supported:

  C:
    Other

  Cc:
    Control

  Cf:
    Format

  Cn:
    Unassigned

  Co:
    Private use

  Cs:
    Surrogate

  L:
    Letter

  Ll:
    Lowercase letter

  Lm:
    Modifier letter

  Lo:
    Other letter

  Lt:
    Title case letter

  Lu:
    Uppercase letter

  M:
    Mark

  Mc:
    Spacing mark

  Me:
    Enclosing mark

  Mn:
    Non-spacing mark

  N:
    Number

  Nd:
    Decimal number

  Nl:
    Letter number

  No:
    Other number

  P:
    Punctuation

  Pc:
    Connector punctuation

  Pd:
    Dash punctuation

  Pe:
    Close punctuation

  Pf:
    Final punctuation

  Pi:
    Initial punctuation

  Po:
    Other punctuation

  Ps:
    Open punctuation

  S:
    Symbol

  Sc:
    Currency symbol

  Sk:
    Modifier symbol

  Sm:
    Mathematical symbol

  So:
    Other symbol

  Z:
    Separator

  Zl:
    Line separator

  Zp:
    Paragraph separator

  Zs:
    Space separator

  The special property L& is also supported. It matches a character
  that has the Lu, Ll, or Lt property, that is, a letter that is not
  classified as a modifier or "other".

  The Cs (Surrogate) property applies only to characters in the
  range U+D800 to U+DFFF. Such characters are invalid in Unicode
  strings and so cannot be tested by PCRE. Perl does not support the
  Cs property.

  The long synonyms for property names supported by Perl (such as
  \p{Letter}) are not supported by PCRE. It is not permitted to
  prefix any of these properties with "Is".

  No character in the Unicode table has the Cn (unassigned)
  property. This property is instead assumed for any code point that
  is not in the Unicode table.

  Specifying caseless matching does not affect these escape
  sequences. For example, \p{Lu} always matches only uppercase
  letters. This is different from the behavior of current versions
  of Perl.

  Matching characters by Unicode property is not fast, as PCRE must
  do a multistage table lookup to find a character property. That is
  why the traditional escape sequences such as \d and \w do not use
  Unicode properties in PCRE by default. However, you can make them
  do so by setting option ucp or by starting the pattern with
  (*UCP).

  Extended Grapheme Clusters

  The \X escape matches any number of Unicode characters that form
  an "extended grapheme cluster", and treats the sequence as an
  atomic group (see below). Up to and including release 8.31, PCRE
  matched an earlier, simpler definition that was equivalent to 
  (?>\PM\pM*). That is, it matched a character without the "mark"
  property, followed by zero or more characters with the "mark"
  property. Characters with the "mark" property are typically
  non-spacing accents that affect the preceding character.

  This simple definition was extended in Unicode to include more
  complicated kinds of composite character by giving each character
  a grapheme breaking property, and creating rules that use these
  properties to define the boundaries of extended grapheme clusters.
  In PCRE releases later than 8.31, \X matches one of these
  clusters.

  \X always matches at least one character. Then it decides whether
  to add more characters according to the following rules for ending
  a cluster:

   • End at the end of the subject string.

   • Do not end between CR and LF; otherwise end after any
     control character.

   • Do not break Hangul (a Korean script) syllable sequences.
     Hangul characters are of five types: L, V, T, LV, and LVT.
     An L character can be followed by an L, V, LV, or LVT
     character. An LV or V character can be followed by a V or T
     character. An LVT or T character can be followed only by a T
     character.

   • Do not end before extending characters or spacing marks.
     Characters with the "mark" property always have the "extend"
     grapheme breaking property.

   • Do not end after prepend characters.

   • Otherwise, end the cluster.

  PCRE Additional Properties

  In addition to the standard Unicode properties described earlier,
  PCRE supports four more that make it possible to convert
  traditional escape sequences, such as \w and \s to use Unicode
  properties. PCRE uses these non-standard, non-Perl properties
  internally when the ucp option is passed. However, they can also
  be used explicitly. The properties are as follows:

  Xan:
    Any alphanumeric character. Matches characters that have
    either the L (letter) or the N (number) property.

  Xps:
    Any Posix space character. Matches the characters tab, line
    feed, vertical tab, form feed, carriage return, and any other
    character that has the Z (separator) property.

  Xsp:
    Any Perl space character. Matches the same as Xps, except that
    vertical tab is excluded.

  Xwd:
    Any Perl "word" character. Matches the same characters as Xan,
    plus underscore.

  Perl and POSIX space are now the same. Perl added VT to its space
  character set at release 5.18 and PCRE changed at release 8.34.

  Xan matches characters that have either the L (letter) or the N
  (number) property. Xps matches the characters tab, linefeed,
  vertical tab, form feed, or carriage return, and any other
  character that has the Z (separator) property. Xsp is the same as
  Xps; it used to exclude vertical tab, for Perl compatibility, but
  Perl changed, and so PCRE followed at release 8.34. Xwd matches
  the same characters as Xan, plus underscore.

  There is another non-standard property, Xuc, which matches any
  character that can be represented by a Universal Character Name in
  C++ and other programming languages. These are the characters $,
  @, ` (grave accent), and all characters with Unicode code points
  >= U+00A0, except for the surrogates U+D800 to U+DFFF. Notice that
  most base (ASCII) characters are excluded. (Universal Character
  Names are of the form \uHHHH or \UHHHHHHHH, where H is a
  hexadecimal digit. Notice that the Xuc property does not match
  these sequences but the characters that they represent.)

  Resetting the Match Start

  The escape sequence \K causes any previously matched characters
  not to be included in the final matched sequence. For example, the
  following pattern matches "foobar", but reports that it has
  matched "bar":

    foo\Kbar

  This feature is similar to a lookbehind assertion (described
  below). However, in this case, the part of the subject before the
  real match does not have to be of fixed length, as lookbehind
  assertions do. The use of \K does not interfere with the setting
  of captured substrings. For example, when the following pattern
  matches "foobar", the first substring is still set to "foo":

    (foo)\Kbar

  Perl documents that the use of \K within assertions is "not well
  defined". In PCRE, \K is acted upon when it occurs inside positive
  assertions, but is ignored in negative assertions. Note that when
  a pattern such as (?=ab\K) matches, the reported start of the
  match can be greater than the end of the match.

  Simple Assertions

  The final use of backslash is for certain simple assertions. An
  assertion specifies a condition that must be met at a particular
  point in a match, without consuming any characters from the
  subject string. The use of subpatterns for more complicated
  assertions is described below. The following are the backslashed
  assertions:

  \b:
    Matches at a word boundary.

  \B:
    Matches when not at a word boundary.

  \A:
    Matches at the start of the subject.

  \Z:
    Matches at the end of the subject, and before a newline at the
    end of the subject.

  \z:
    Matches only at the end of the subject.

  \G:
    Matches at the first matching position in the subject.

  Inside a character class, \b has a different meaning; it matches
  the backspace character. If any other of these assertions appears
  in a character class, by default it matches the corresponding
  literal character (for example, \B matches the letter B).

  A word boundary is a position in the subject string where the
  current character and the previous character do not both match \w
  or \W (that is, one matches \w and the other matches \W), or the
  start or end of the string if the first or last character matches
  \w, respectively. In UTF mode, the meanings of \w and \W can be
  changed by setting option ucp. When this is done, it also
  affects \b and \B. PCRE and Perl do not have a separate "start of
  word" or "end of word" metasequence. However, whatever follows \b
  normally determines which it is. For example, the fragment \ba
  matches "a" at the start of a word.

  The \A, \Z, and \z assertions differ from the traditional
  circumflex and dollar (described in the next section) in that they
  only ever match at the very start and end of the subject string,
  whatever options are set. Thus, they are independent of multiline
  mode. These three assertions are not affected by options notbol
  or noteol, which affect only the behavior of the circumflex and
  dollar metacharacters. However, if argument startoffset of 
  run/3 is non-zero, indicating that matching is to start at a
  point other than the beginning of the subject, \A can never match.
  The difference between \Z and \z is that \Z matches before a
  newline at the end of the string and at the very end, while \z
  matches only at the end.

  The \G assertion is true only when the current matching position
  is at the start point of the match, as specified by argument 
  startoffset of run/3. It differs from \A when the value of 
  startoffset is non-zero. By calling run/3 multiple times with
  appropriate arguments, you can mimic the Perl option /g, and it
  is in this kind of implementation where \G can be useful.

  Notice, however, that the PCRE interpretation of \G, as the start
  of the current match, is subtly different from Perl, which defines
  it as the end of the previous match. In Perl, these can be
  different when the previously matched string was empty. As PCRE
  does only one match at a time, it cannot reproduce this behavior.

  If all the alternatives of a pattern begin with \G, the expression
  is anchored to the starting match position, and the "anchored"
  flag is set in the compiled regular expression.

Circumflex and Dollar

  The circumflex and dollar metacharacters are zero-width
  assertions. That is, they test for a particular condition to be
  true without consuming any characters from the subject string.

  Outside a character class, in the default matching mode, the
  circumflex character is an assertion that is true only if the
  current matching point is at the start of the subject string. If
  argument startoffset of run/3 is non-zero, circumflex can
  never match if option multiline is unset. Inside a character
  class, circumflex has an entirely different meaning (see below).

  Circumflex needs not to be the first character of the pattern if
  some alternatives are involved, but it is to be the first thing in
  each alternative in which it appears if the pattern is ever to
  match that branch. If all possible alternatives start with a
  circumflex, that is, if the pattern is constrained to match only
  at the start of the subject, it is said to be an "anchored"
  pattern. (There are also other constructs that can cause a pattern
  to be anchored.)

  The dollar character is an assertion that is true only if the
  current matching point is at the end of the subject string, or
  immediately before a newline at the end of the string (by
  default). Notice however that it does not match the newline.
  Dollar needs not to be the last character of the pattern if some
  alternatives are involved, but it is to be the last item in any
  branch in which it appears. Dollar has no special meaning in a
  character class.

  The meaning of dollar can be changed so that it matches only at
  the very end of the string, by setting option dollar_endonly at
  compile time. This does not affect the \Z assertion.

  The meanings of the circumflex and dollar characters are changed
  if option multiline is set. When this is the case, a circumflex
  matches immediately after internal newlines and at the start of
  the subject string. It does not match after a newline that ends
  the string. A dollar matches before any newlines in the string,
  and at the very end, when multiline is set. When newline is
  specified as the two-character sequence CRLF, isolated CR and LF
  characters do not indicate newlines.

  For example, the pattern /^abc$/ matches the subject string
  "def\nabc" (where \n represents a newline) in multiline mode, but
  not otherwise. So, patterns that are anchored in single-line mode
  because all branches start with ^ are not anchored in multiline
  mode, and a match for circumflex is possible when argument 
  startoffset of run/3 is non-zero. Option dollar_endonly is
  ignored if multiline is set.

  Notice that the sequences \A, \Z, and \z can be used to match the
  start and end of the subject in both modes. If all branches of a
  pattern start with \A, it is always anchored, regardless if 
  multiline is set.

Full Stop (Period, Dot) and \N

  Outside a character class, a dot in the pattern matches any
  character in the subject string except (by default) a character
  that signifies the end of a line.

  When a line ending is defined as a single character, dot never
  matches that character. When the two-character sequence CRLF is
  used, dot does not match CR if it is immediately followed by LF,
  otherwise it matches all characters (including isolated CRs and
  LFs). When any Unicode line endings are recognized, dot does not
  match CR, LF, or any of the other line-ending characters.

  The behavior of dot regarding newlines can be changed. If option 
  dotall is set, a dot matches any character, without exception. If
  the two-character sequence CRLF is present in the subject string,
  it takes two dots to match it.

  The handling of dot is entirely independent of the handling of
  circumflex and dollar, the only relationship is that both involve
  newlines. Dot has no special meaning in a character class.

  The escape sequence \N behaves like a dot, except that it is not
  affected by option PCRE_DOTALL. That is, it matches any
  character except one that signifies the end of a line. Perl also
  uses \N to match characters by name but PCRE does not support
  this.

Matching a Single Data Unit

  Outside a character class, the escape sequence \C matches any data
  unit, regardless if a UTF mode is set. One data unit is one byte.
  Unlike a dot, \C always matches line-ending characters. The
  feature is provided in Perl to match individual bytes in UTF-8
  mode, but it is unclear how it can usefully be used. As \C breaks
  up characters into individual data units, matching one unit with
  \C in a UTF mode means that the remaining string can start with a
  malformed UTF character. This has undefined results, as PCRE
  assumes that it deals with valid UTF strings.

  PCRE does not allow \C to appear in lookbehind assertions
  (described below) in a UTF mode, as this would make it impossible
  to calculate the length of the lookbehind.

  The \C escape sequence is best avoided. However, one way of using
  it that avoids the problem of malformed UTF characters is to use a
  lookahead to check the length of the next character, as in the
  following pattern, which can be used with a UTF-8 string (ignore
  whitespace and line breaks):

    (?| (?=[\x00-\x7f])(\C) |
        (?=[\x80-\x{7ff}])(\C)(\C) |
        (?=[\x{800}-\x{ffff}])(\C)(\C)(\C) |
        (?=[\x{10000}-\x{1fffff}])(\C)(\C)(\C)(\C))

  A group that starts with (?| resets the capturing parentheses
  numbers in each alternative (see section Duplicate Subpattern
  Numbers). The assertions at the start of each branch check the
  next UTF-8 character for values whose encoding uses 1, 2, 3, or 4
  bytes, respectively. The individual bytes of the character are
  then captured by the appropriate number of groups.

Square Brackets and Character Classes

  An opening square bracket introduces a character class, terminated
  by a closing square bracket. A closing square bracket on its own
  is not special by default. However, if option 
  PCRE_JAVASCRIPT_COMPAT is set, a lone closing square bracket
  causes a compile-time error. If a closing square bracket is
  required as a member of the class, it is to be the first data
  character in the class (after an initial circumflex, if present)
  or escaped with a backslash.

  A character class matches a single character in the subject. In a
  UTF mode, the character can be more than one data unit long. A
  matched character must be in the set of characters defined by the
  class, unless the first character in the class definition is a
  circumflex, in which case the subject character must not be in the
  set defined by the class. If a circumflex is required as a member
  of the class, ensure that it is not the first character, or escape
  it with a backslash.

  For example, the character class [aeiou] matches any lowercase
  vowel, while [^aeiou] matches any character that is not a
  lowercase vowel. Notice that a circumflex is just a convenient
  notation for specifying the characters that are in the class by
  enumerating those that are not. A class that starts with a
  circumflex is not an assertion; it still consumes a character from
  the subject string, and therefore it fails if the current pointer
  is at the end of the string.

  In UTF-8 mode, characters with values > 255 (0xffff) can be
  included in a class as a literal string of data units, or by using
  the \x{ escaping mechanism.

  When caseless matching is set, any letters in a class represent
  both their uppercase and lowercase versions. For example, a
  caseless [aeiou] matches "A" and "a", and a caseless [^aeiou]
  does not match "A", but a caseful version would. In a UTF mode,
  PCRE always understands the concept of case for characters whose
  values are < 256, so caseless matching is always possible. For
  characters with higher values, the concept of case is supported
  only if PCRE is compiled with Unicode property support. If you
  want to use caseless matching in a UTF mode for characters >=,
  ensure that PCRE is compiled with Unicode property support and
  with UTF support.

  Characters that can indicate line breaks are never treated in any
  special way when matching character classes, whatever line-ending
  sequence is in use, and whatever setting of options PCRE_DOTALL
  and PCRE_MULTILINE is used. A class such as [^a] always matches
  one of these characters.

  The minus (hyphen) character can be used to specify a range of
  characters in a character class. For example, [d-m] matches any
  letter between d and m, inclusive. If a minus character is
  required in a class, it must be escaped with a backslash or appear
  in a position where it cannot be interpreted as indicating a
  range, typically as the first or last character in the class, or
  immediately after a range. For example, [b-d-z] matches letters in
  the range b to d, a hyphen character, or z.

  The literal character "]" cannot be the end character of a range.
  A pattern such as [W-]46] is interpreted as a class of two
  characters ("W" and "-") followed by a literal string "46]", so it
  would match "W46]" or "-46]". However, if "]" is escaped with a
  backslash, it is interpreted as the end of range, so [W-\]46] is
  interpreted as a class containing a range followed by two other
  characters. The octal or hexadecimal representation of "]" can
  also be used to end a range.

  An error is generated if a POSIX character class (see below) or an
  escape sequence other than one that defines a single character
  appears at a point where a range ending character is expected. For
  example, [z-\xff] is valid, but [A-\d] and [A-[:digit:]] are not.

  Ranges operate in the collating sequence of character values. They
  can also be used for characters specified numerically, for
  example, [\000-\037]. Ranges can include any characters that are
  valid for the current mode.

  If a range that includes letters is used when caseless matching is
  set, it matches the letters in either case. For example, [W-c] is
  equivalent to [][\\^_`wxyzabc], matched caselessly. In a non-UTF
  mode, if character tables for a French locale are in use,
  [\xc8-\xcb] matches accented E characters in both cases. In UTF
  modes, PCRE supports the concept of case for characters with
  values > 255 only when it is compiled with Unicode property
  support.

  The character escape sequences \d, \D, \h, \H, \p, \P, \s, \S, \v,
  \V, \w, and \W can appear in a character class, and add the
  characters that they match to the class. For example, [\dABCDEF]
  matches any hexadecimal digit. In UTF modes, option ucp affects
  the meanings of \d, \s, \w and their uppercase partners, just as
  it does when they appear outside a character class, as described
  in section Generic Character Types earlier. The escape sequence
  \b has a different meaning inside a character class; it matches
  the backspace character. The sequences \B, \N, \R, and \X are not
  special inside a character class. Like any other unrecognized
  escape sequences, they are treated as the literal characters "B",
  "N", "R", and "X".

  A circumflex can conveniently be used with the uppercase character
  types to specify a more restricted set of characters than the
  matching lowercase type. For example, class [^\W_] matches any
  letter or digit, but not underscore, while [\w] includes
  underscore. A positive character class is to be read as "something
  OR something OR ..." and a negative class as "NOT something AND
  NOT something AND NOT ...".

  Only the following metacharacters are recognized in character
  classes:

   • Backslash

   • Hyphen (only where it can be interpreted as specifying a
     range)

   • Circumflex (only at the start)

   • Opening square bracket (only when it can be interpreted as
     introducing a Posix class name, or for a special
     compatibility feature; see the next two sections)

   • Terminating closing square bracket

  However, escaping other non-alphanumeric characters does no harm.

Posix Character Classes

  Perl supports the Posix notation for character classes. This uses
  names enclosed by [: and :] within the enclosing square brackets.
  PCRE also supports this notation. For example, the following
  matches "0", "1", any alphabetic character, or "%":

    [01[:alpha:]%]

  The following are the supported class names:

  alnum:
    Letters and digits

  alpha:
    Letters

  blank:
    Space or tab only

  cntrl:
    Control characters

  digit:
    Decimal digits (same as \d)

  graph:
    Printing characters, excluding space

  lower:
    Lowercase letters

  print:
    Printing characters, including space

  punct:
    Printing characters, excluding letters, digits, and space

  space:
    Whitespace (the same as \s from PCRE 8.34)

  upper:
    Uppercase letters

  word:
    "Word" characters (same as \w)

  xdigit:
    Hexadecimal digits

  There is another character class, ascii, that erroneously
  matches Latin-1 characters instead of the 0-127 range specified by
  POSIX. This cannot be fixed without altering the behaviour of
  other classes, so we recommend matching the range with [\\0-\x7f]
  instead.

  The default "space" characters are HT (9), LF (10), VT (11), FF
  (12), CR (13), and space (32). If locale-specific matching is
  taking place, the list of space characters may be different; there
  may be fewer or more of them. "Space" used to be different to \s,
  which did not include VT, for Perl compatibility. However, Perl
  changed at release 5.18, and PCRE followed at release 8.34.
  "Space" and \s now match the same set of characters.

  The name "word" is a Perl extension, and "blank" is a GNU
  extension from Perl 5.8. Another Perl extension is negation, which
  is indicated by a ^ character after the colon. For example, the
  following matches "1", "2", or any non-digit:

    [12[:^digit:]]

  PCRE (and Perl) also recognize the Posix syntax [.ch.] and [=ch=]
  where "ch" is a "collating element", but these are not supported,
  and an error is given if they are encountered.

  By default, characters with values > 255 do not match any of the
  Posix character classes. However, if option PCRE_UCP is passed
  to pcre_compile(), some of the classes are changed so that
  Unicode character properties are used. This is achieved by
  replacing certain Posix classes by other sequences, as follows:

  [:alnum:]:
    Becomes \p{Xan}

  [:alpha:]:
    Becomes \p{L}

  [:blank:]:
    Becomes \h

  [:digit:]:
    Becomes \p{Nd}

  [:lower:]:
    Becomes \p{Ll}

  [:space:]:
    Becomes \p{Xps}

  [:upper:]:
    Becomes \p{Lu}

  [:word:]:
    Becomes \p{Xwd}

  Negated versions, such as [:^alpha:], use \P instead of \p. Three
  other POSIX classes are handled specially in UCP mode:

  [:graph:]:
    This matches characters that have glyphs that mark the page
    when printed. In Unicode property terms, it matches all
    characters with the L, M, N, P, S, or Cf properties, except
    for:

    U+061C:
      Arabic Letter Mark

    U+180E:
      Mongolian Vowel Separator

    U+2066 - U+2069:
      Various "isolate"s

  [:print:]:
    This matches the same characters as [:graph:] plus space
    characters that are not controls, that is, characters with the
    Zs property.

  [:punct:]:
    This matches all characters that have the Unicode P
    (punctuation) property, plus those characters whose code
    points are less than 128 that have the S (Symbol) property.

  The other POSIX classes are unchanged, and match only characters
  with code points less than 128.

  Compatibility Feature for Word Boundaries

  In the POSIX.2 compliant library that was included in 4.4BSD Unix,
  the ugly syntax [[:<:]] and [[:>:]] is used for matching "start of
  word" and "end of word". PCRE treats these items as follows:

  [[:<:]]:
    is converted to \b(?=\w)

  [[:>:]]:
    is converted to \b(?<=\w)

  Only these exact character sequences are recognized. A sequence
  such as [a[:<:]b] provokes error for an unrecognized POSIX class
  name. This support is not compatible with Perl. It is provided to
  help migrations from other environments, and is best not used in
  any new patterns. Note that \b matches at the start and the end of
  a word (see "Simple assertions" above), and in a Perl-style
  pattern the preceding or following character normally shows which
  is wanted, without the need for the assertions that are used above
  in order to give exactly the POSIX behaviour.

Vertical Bar

  Vertical bar characters are used to separate alternative patterns.
  For example, the following pattern matches either "gilbert" or
  "sullivan":

    gilbert|sullivan

  Any number of alternatives can appear, and an empty alternative is
  permitted (matching the empty string). The matching process tries
  each alternative in turn, from left to right, and the first that
  succeeds is used. If the alternatives are within a subpattern
  (defined in section Subpatterns), "succeeds" means matching the
  remaining main pattern and the alternative in the subpattern.

Internal Option Setting

  The settings of the Perl-compatible options caseless, multiline, 
  dotall, and extended can be changed from within the pattern by
  a sequence of Perl option letters enclosed between "(?" and ")".
  The option letters are as follows:

  i:
    For caseless

  m:
    For multiline

  s:
    For dotall

  x:
    For extended

  For example, (?im) sets caseless, multiline matching. These
  options can also be unset by preceding the letter with a hyphen. A
  combined setting and unsetting such as (?im-sx), which sets 
  caseless and multiline, while unsetting dotall and extended,
  is also permitted. If a letter appears both before and after the
  hyphen, the option is unset.

  The PCRE-specific options dupnames, ungreedy, and extra can
  be changed in the same way as the Perl-compatible options by using
  the characters J, U, and X respectively.

  When one of these option changes occurs at top-level (that is, not
  inside subpattern parentheses), the change applies to the
  remainder of the pattern that follows.

  An option change within a subpattern (see section Subpatterns)
  affects only that part of the subpattern that follows it. So, the
  following matches abc and aBc and no other strings (assuming 
  caseless is not used):

    (a(?i)b)c

  By this means, options can be made to have different settings in
  different parts of the pattern. Any changes made in one
  alternative do carry on into subsequent branches within the same
  subpattern. For example:

    (a(?i)b|c)

  matches "ab", "aB", "c", and "C", although when matching "C" the
  first branch is abandoned before the option setting. This is
  because the effects of option settings occur at compile time.
  There would be some weird behavior otherwise.

  Note:
    Other PCRE-specific options can be set by the application when
    the compiling or matching functions are called. Sometimes the
    pattern can contain special leading sequences, such as
    (*CRLF), to override what the application has set or what has
    been defaulted. Details are provided in section Newline
    Sequences earlier.

    The (*UTF8) and (*UCP) leading sequences can be used to set
    UTF and Unicode property modes. They are equivalent to setting
    options unicode and ucp, respectively. The (*UTF) sequence
    is a generic version that can be used with any of the
    libraries. However, the application can set option never_utf,
    which locks out the use of the (*UTF) sequences.

Subpatterns

  Subpatterns are delimited by parentheses (round brackets), which
  can be nested. Turning part of a pattern into a subpattern does
  two things:

  1.:
    It localizes a set of alternatives. For example, the following
    pattern matches "cataract", "caterpillar", or "cat":

      cat(aract|erpillar|)

    Without the parentheses, it would match "cataract",
    "erpillar", or an empty string.

  2.:
    It sets up the subpattern as a capturing subpattern. That is,
    when the complete pattern matches, that portion of the subject
    string that matched the subpattern is passed back to the
    caller through the return value of run/3.

  Opening parentheses are counted from left to right (starting from
  1) to obtain numbers for the capturing subpatterns. For example,
  if the string "the red king" is matched against the following
  pattern, the captured substrings are "red king", "red", and
  "king", and are numbered 1, 2, and 3, respectively:

    the ((red|white) (king|queen))

  It is not always helpful that plain parentheses fulfill two
  functions. Often a grouping subpattern is required without a
  capturing requirement. If an opening parenthesis is followed by a
  question mark and a colon, the subpattern does not do any
  capturing, and is not counted when computing the number of any
  subsequent capturing subpatterns. For example, if the string "the
  white queen" is matched against the following pattern, the
  captured substrings are "white queen" and "queen", and are
  numbered 1 and 2:

    the ((?:red|white) (king|queen))

  The maximum number of capturing subpatterns is 65535.

  As a convenient shorthand, if any option settings are required at
  the start of a non-capturing subpattern, the option letters can
  appear between "?" and ":". Thus, the following two patterns match
  the same set of strings:

    (?i:saturday|sunday)
    (?:(?i)saturday|sunday)

  As alternative branches are tried from left to right, and options
  are not reset until the end of the subpattern is reached, an
  option setting in one branch does affect subsequent branches, so
  the above patterns match both "SUNDAY" and "Saturday".

Duplicate Subpattern Numbers

  Perl 5.10 introduced a feature where each alternative in a
  subpattern uses the same numbers for its capturing parentheses.
  Such a subpattern starts with (?| and is itself a non-capturing
  subpattern. For example, consider the following pattern:

    (?|(Sat)ur|(Sun))day

  As the two alternatives are inside a (?| group, both sets of
  capturing parentheses are numbered one. Thus, when the pattern
  matches, you can look at captured substring number one, whichever
  alternative matched. This construct is useful when you want to
  capture a part, but not all, of one of many alternatives. Inside a 
  (?| group, parentheses are numbered as usual, but the number is
  reset at the start of each branch. The numbers of any capturing
  parentheses that follow the subpattern start after the highest
  number used in any branch. The following example is from the Perl
  documentation; the numbers underneath show in which buffer the
  captured content is stored:

    # before  ---------------branch-reset----------- after
    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
    # 1            2         2  3        2     3     4

  A back reference to a numbered subpattern uses the most recent
  value that is set for that number by any subpattern. The following
  pattern matches "abcabc" or "defdef":

    /(?|(abc)|(def))\1/

  In contrast, a subroutine call to a numbered subpattern always
  refers to the first one in the pattern with the given number. The
  following pattern matches "abcabc" or "defabc":

    /(?|(abc)|(def))(?1)/

  If a condition test for a subpattern having matched refers to a
  non-unique number, the test is true if any of the subpatterns of
  that number have matched.

  An alternative approach using this "branch reset" feature is to
  use duplicate named subpatterns, as described in the next section.

Named Subpatterns

  Identifying capturing parentheses by number is simple, but it can
  be hard to keep track of the numbers in complicated regular
  expressions. Also, if an expression is modified, the numbers can
  change. To help with this difficulty, PCRE supports the naming of
  subpatterns. This feature was not added to Perl until release
  5.10. Python had the feature earlier, and PCRE introduced it at
  release 4.0, using the Python syntax. PCRE now supports both the
  Perl and the Python syntax. Perl allows identically numbered
  subpatterns to have different names, but PCRE does not.

  In PCRE, a subpattern can be named in one of three ways: 
  (?<name>...) or (?'name'...) as in Perl, or (?P<name>...) as
  in Python. References to capturing parentheses from other parts of
  the pattern, such as back references, recursion, and conditions,
  can be made by name and by number.

  Names consist of up to 32 alphanumeric characters and underscores,
  but must start with a non-digit. Named capturing parentheses are
  still allocated numbers as well as names, exactly as if the names
  were not present. The capture specification to run/3 can use
  named values if they are present in the regular expression.

  By default, a name must be unique within a pattern, but this
  constraint can be relaxed by setting option dupnames at compile
  time. (Duplicate names are also always permitted for subpatterns
  with the same number, set up as described in the previous
  section.) Duplicate names can be useful for patterns where only
  one instance of the named parentheses can match. Suppose that you
  want to match the name of a weekday, either as a 3-letter
  abbreviation or as the full name, and in both cases you want to
  extract the abbreviation. The following pattern (ignoring the line
  breaks) does the job:

    (?<DN>Mon|Fri|Sun)(?:day)?|
    (?<DN>Tue)(?:sday)?|
    (?<DN>Wed)(?:nesday)?|
    (?<DN>Thu)(?:rsday)?|
    (?<DN>Sat)(?:urday)?

  There are five capturing substrings, but only one is ever set
  after a match. (An alternative way of solving this problem is to
  use a "branch reset" subpattern, as described in the previous
  section.)

  For capturing named subpatterns which names are not unique, the
  first matching occurrence (counted from left to right in the
  subject) is returned from run/3, if the name is specified in the 
  values part of the capture statement. The all_names capturing
  value matches all the names in the same way.

  Note:
    You cannot use different names to distinguish between two
    subpatterns with the same number, as PCRE uses only the
    numbers when matching. For this reason, an error is given at
    compile time if different names are specified to subpatterns
    with the same number. However, you can specify the same name
    to subpatterns with the same number, even when dupnames is
    not set.

Repetition

  Repetition is specified by quantifiers, which can follow any of
  the following items:

   • A literal data character

   • The dot metacharacter

   • The \C escape sequence

   • The \X escape sequence

   • The \R escape sequence

   • An escape such as \d or \pL that matches a single character

   • A character class

   • A back reference (see the next section)

   • A parenthesized subpattern (including assertions)

   • A subroutine call to a subpattern (recursive or otherwise)

  The general repetition quantifier specifies a minimum and maximum
  number of permitted matches, by giving the two numbers in curly
  brackets (braces), separated by a comma. The numbers must be <
  65536, and the first must be less than or equal to the second. For
  example, the following matches "zz", "zzz", or "zzzz":

    z{2,4}

  A closing brace on its own is not a special character. If the
  second number is omitted, but the comma is present, there is no
  upper limit. If the second number and the comma are both omitted,
  the quantifier specifies an exact number of required matches.
  Thus, the following matches at least three successive vowels, but
  can match many more:

    [aeiou]{3,}

  The following matches exactly eight digits:

    \d{8}

  An opening curly bracket that appears in a position where a
  quantifier is not allowed, or one that does not match the syntax
  of a quantifier, is taken as a literal character. For example,
  {,6} is not a quantifier, but a literal string of four characters.

  In Unicode mode, quantifiers apply to characters rather than to
  individual data units. Thus, for example, \x{100}{2} matches two
  characters, each of which is represented by a 2-byte sequence in a
  UTF-8 string. Similarly, \X{3} matches three Unicode extended
  grapheme clusters, each of which can be many data units long (and
  they can be of different lengths).

  The quantifier {0} is permitted, causing the expression to behave
  as if the previous item and the quantifier were not present. This
  can be useful for subpatterns that are referenced as subroutines
  from elsewhere in the pattern (but see also section Defining
  Subpatterns for Use by Reference Only). Items other than
  subpatterns that have a {0} quantifier are omitted from the
  compiled pattern.

  For convenience, the three most common quantifiers have
  single-character abbreviations:

  *:
    Equivalent to {0,}

  +:
    Equivalent to {1,}

  ?:
    Equivalent to {0,1}

  Infinite loops can be constructed by following a subpattern that
  can match no characters with a quantifier that has no upper limit,
  for example:

    (a?)*

  Earlier versions of Perl and PCRE used to give an error at compile
  time for such patterns. However, as there are cases where this can
  be useful, such patterns are now accepted. However, if any
  repetition of the subpattern matches no characters, the loop is
  forcibly broken.

  By default, the quantifiers are "greedy", that is, they match as
  much as possible (up to the maximum number of permitted times),
  without causing the remaining pattern to fail. The classic example
  of where this gives problems is in trying to match comments in C
  programs. These appear between /* and */. Within the comment,
  individual * and / characters can appear. An attempt to match C
  comments by applying the pattern

    /\*.*\*/

  to the string

    /* first comment */  not comment  /* second comment */

  fails, as it matches the entire string owing to the greediness of
  the .* item.

  However, if a quantifier is followed by a question mark, it ceases
  to be greedy, and instead matches the minimum number of times
  possible, so the following pattern does the right thing with the C
  comments:

    /\*.*?\*/

  The meaning of the various quantifiers is not otherwise changed,
  only the preferred number of matches. Do not confuse this use of
  question mark with its use as a quantifier in its own right. As it
  has two uses, it can sometimes appear doubled, as in

    \d??\d

  which matches one digit by preference, but can match two if that
  is the only way the remaining pattern matches.

  If option ungreedy is set (an option that is not available in
  Perl), the quantifiers are not greedy by default, but individual
  ones can be made greedy by following them with a question mark.
  That is, it inverts the default behavior.

  When a parenthesized subpattern is quantified with a minimum
  repeat count that is > 1 or with a limited maximum, more memory is
  required for the compiled pattern, in proportion to the size of
  the minimum or maximum.

  If a pattern starts with .* or .{0,} and option dotall
  (equivalent to Perl option /s) is set, thus allowing the dot to
  match newlines, the pattern is implicitly anchored, because
  whatever follows is tried against every character position in the
  subject string. So, there is no point in retrying the overall
  match at any position after the first. PCRE normally treats such a
  pattern as if it was preceded by \A.

  In cases where it is known that the subject string contains no
  newlines, it is worth setting dotall to obtain this
  optimization, or alternatively using ^ to indicate anchoring
  explicitly.

  However, there are some cases where the optimization cannot be
  used. When .* is inside capturing parentheses that are the subject
  of a back reference elsewhere in the pattern, a match at the start
  can fail where a later one succeeds. Consider, for example:

    (.*)abc\1

  If the subject is "xyz123abc123", the match point is the fourth
  character. Therefore, such a pattern is not implicitly anchored.

  Another case where implicit anchoring is not applied is when the
  leading .* is inside an atomic group. Once again, a match at the
  start can fail where a later one succeeds. Consider the following
  pattern:

    (?>.*?a)b

  It matches "ab" in the subject "aab". The use of the backtracking
  control verbs (*PRUNE) and (*SKIP) also disable this optimization.

  When a capturing subpattern is repeated, the value captured is the
  substring that matched the final iteration. For example, after

    (tweedle[dume]{3}\s*)+

  has matched "tweedledum tweedledee", the value of the captured
  substring is "tweedledee". However, if there are nested capturing
  subpatterns, the corresponding captured values can have been set
  in previous iterations. For example, after

    /(a|(b))+/

  matches "aba", the value of the second captured substring is "b".

Atomic Grouping and Possessive Quantifiers

  With both maximizing ("greedy") and minimizing ("ungreedy" or
  "lazy") repetition, failure of what follows normally causes the
  repeated item to be re-evaluated to see if a different number of
  repeats allows the remaining pattern to match. Sometimes it is
  useful to prevent this, either to change the nature of the match,
  or to cause it to fail earlier than it otherwise might, when the
  author of the pattern knows that there is no point in carrying on.

  Consider, for example, the pattern \d+foo when applied to the
  following subject line:

    123456bar

  After matching all six digits and then failing to match "foo", the
  normal action of the matcher is to try again with only five digits
  matching item \d+, and then with four, and so on, before
  ultimately failing. "Atomic grouping" (a term taken from Jeffrey
  Friedl's book) provides the means for specifying that once a
  subpattern has matched, it is not to be re-evaluated in this way.

  If atomic grouping is used for the previous example, the matcher
  gives up immediately on failing to match "foo" the first time. The
  notation is a kind of special parenthesis, starting with (?> as
  in the following example:

    (?>\d+)foo

  This kind of parenthesis "locks up" the part of the pattern it
  contains once it has matched, and a failure further into the
  pattern is prevented from backtracking into it. Backtracking past
  it to previous items, however, works as normal.

  An alternative description is that a subpattern of this type
  matches the string of characters that an identical standalone
  pattern would match, if anchored at the current point in the
  subject string.

  Atomic grouping subpatterns are not capturing subpatterns. Simple
  cases such as the above example can be thought of as a maximizing
  repeat that must swallow everything it can. So, while both \d+ and
  \d+? are prepared to adjust the number of digits they match to
  make the remaining pattern match, (?>\d+) can only match an
  entire sequence of digits.

  Atomic groups in general can contain any complicated subpatterns,
  and can be nested. However, when the subpattern for an atomic
  group is just a single repeated item, as in the example above, a
  simpler notation, called a "possessive quantifier" can be used.
  This consists of an extra + character following a quantifier.
  Using this notation, the previous example can be rewritten as

    \d++foo

  Notice that a possessive quantifier can be used with an entire
  group, for example:

    (abc|xyz){2,3}+

  Possessive quantifiers are always greedy; the setting of option 
  ungreedy is ignored. They are a convenient notation for the
  simpler forms of an atomic group. However, there is no difference
  in the meaning of a possessive quantifier and the equivalent
  atomic group, but there can be a performance difference;
  possessive quantifiers are probably slightly faster.

  The possessive quantifier syntax is an extension to the Perl 5.8
  syntax. Jeffrey Friedl originated the idea (and the name) in the
  first edition of his book. Mike McCloskey liked it, so implemented
  it when he built the Sun Java package, and PCRE copied it from
  there. It ultimately found its way into Perl at release 5.10.

  PCRE has an optimization that automatically "possessifies" certain
  simple pattern constructs. For example, the sequence A+B is
  treated as A++B, as there is no point in backtracking into a
  sequence of A:s when B must follow.

  When a pattern contains an unlimited repeat inside a subpattern
  that can itself be repeated an unlimited number of times, the use
  of an atomic group is the only way to avoid some failing matches
  taking a long time. The pattern

    (\D+|<\d+>)*[!?]

  matches an unlimited number of substrings that either consist of
  non-digits, or digits enclosed in <>, followed by ! or ?. When it
  matches, it runs quickly. However, if it is applied to

    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa

  it takes a long time before reporting failure. This is because the
  string can be divided between the internal \D+ repeat and the
  external * repeat in many ways, and all must be tried. (The
  example uses [!?] rather than a single character at the end, as
  both PCRE and Perl have an optimization that allows for fast
  failure when a single character is used. They remember the last
  single character that is required for a match, and fail early if
  it is not present in the string.) If the pattern is changed so
  that it uses an atomic group, like the following, sequences of
  non-digits cannot be broken, and failure happens quickly:

    ((?>\D+)|<\d+>)*[!?]

Back References

  Outside a character class, a backslash followed by a digit > 0
  (and possibly further digits) is a back reference to a capturing
  subpattern earlier (that is, to its left) in the pattern, provided
  there have been that many previous capturing left parentheses.

  However, if the decimal number following the backslash is < 10, it
  is always taken as a back reference, and causes an error only if
  there are not that many capturing left parentheses in the entire
  pattern. That is, the parentheses that are referenced do need not
  be to the left of the reference for numbers < 10. A "forward back
  reference" of this type can make sense when a repetition is
  involved and the subpattern to the right has participated in an
  earlier iteration.

  It is not possible to have a numerical "forward back reference" to
  a subpattern whose number is 10 or more using this syntax, as a
  sequence such as \50 is interpreted as a character defined in
  octal. For more details of the handling of digits following a
  backslash, see section Non-Printing Characters earlier. There is
  no such problem when named parentheses are used. A back reference
  to any subpattern is possible using named parentheses (see below).

  Another way to avoid the ambiguity inherent in the use of digits
  following a backslash is to use the \g escape sequence. This
  escape must be followed by an unsigned number or a negative
  number, optionally enclosed in braces. The following examples are
  identical:

    (ring), \1
    (ring), \g1
    (ring), \g{1}

  An unsigned number specifies an absolute reference without the
  ambiguity that is present in the older syntax. It is also useful
  when literal digits follow the reference. A negative number is a
  relative reference. Consider the following example:

    (abc(def)ghi)\g{-1}

  The sequence \g{-1} is a reference to the most recently started
  capturing subpattern before \g, that is, it is equivalent to \2 in
  this example. Similarly, \g{-2} would be equivalent to \1. The use
  of relative references can be helpful in long patterns, and also
  in patterns that are created by joining fragments containing
  references within themselves.

  A back reference matches whatever matched the capturing subpattern
  in the current subject string, rather than anything matching the
  subpattern itself (section Subpattern as Subroutines describes a
  way of doing that). So, the following pattern matches "sense and
  sensibility" and "response and responsibility", but not "sense and
  responsibility":

    (sens|respons)e and \1ibility

  If caseful matching is in force at the time of the back reference,
  the case of letters is relevant. For example, the following
  matches "rah rah" and "RAH RAH", but not "RAH rah", although the
  original capturing subpattern is matched caselessly:

    ((?i)rah)\s+\1

  There are many different ways of writing back references to named
  subpatterns. The .NET syntax \k{name} and the Perl syntax 
  \k<name> or \k'name' are supported, as is the Python syntax 
  (?P=name). The unified back reference syntax in Perl 5.10, in
  which \g can be used for both numeric and named references, is
  also supported. The previous example can be rewritten in the
  following ways:

    (?<p1>(?i)rah)\s+\k<p1>
    (?'p1'(?i)rah)\s+\k{p1}
    (?P<p1>(?i)rah)\s+(?P=p1)
    (?<p1>(?i)rah)\s+\g{p1}

  A subpattern that is referenced by name can appear in the pattern
  before or after the reference.

  There can be more than one back reference to the same subpattern.
  If a subpattern has not been used in a particular match, any back
  references to it always fails. For example, the following pattern
  always fails if it starts to match "a" rather than "bc":

    (a|(bc))\2

  As there can be many capturing parentheses in a pattern, all
  digits following the backslash are taken as part of a potential
  back reference number. If the pattern continues with a digit
  character, some delimiter must be used to terminate the back
  reference. If option extended is set, this can be whitespace.
  Otherwise an empty comment (see section Comments) can be used.

  Recursive Back References

  A back reference that occurs inside the parentheses to which it
  refers fails when the subpattern is first used, so, for example,
  (a\1) never matches. However, such references can be useful inside
  repeated subpatterns. For example, the following pattern matches
  any number of "a"s and also "aba", "ababbaa", and so on:

    (a|b\1)+

  At each iteration of the subpattern, the back reference matches
  the character string corresponding to the previous iteration. In
  order for this to work, the pattern must be such that the first
  iteration does not need to match the back reference. This can be
  done using alternation, as in the example above, or by a
  quantifier with a minimum of zero.

  Back references of this type cause the group that they reference
  to be treated as an atomic group. Once the whole group has been
  matched, a subsequent matching failure cannot cause backtracking
  into the middle of the group.

Assertions

  An assertion is a test on the characters following or preceding
  the current matching point that does not consume any characters.
  The simple assertions coded as \b, \B, \A, \G, \Z, \z, ^, and $
  are described in the previous sections.

  More complicated assertions are coded as subpatterns. There are
  two kinds: those that look ahead of the current position in the
  subject string, and those that look behind it. An assertion
  subpattern is matched in the normal way, except that it does not
  cause the current matching position to be changed.

  Assertion subpatterns are not capturing subpatterns. If such an
  assertion contains capturing subpatterns within it, these are
  counted for the purposes of numbering the capturing subpatterns in
  the whole pattern. However, substring capturing is done only for
  positive assertions. (Perl sometimes, but not always, performs
  capturing in negative assertions.)

  Warning:
    If a positive assertion containing one or more capturing
    subpatterns succeeds, but failure to match later in the
    pattern causes backtracking over this assertion, the captures
    within the assertion are reset only if no higher numbered
    captures are already set. This is, unfortunately, a
    fundamental limitation of the current implementation, and as
    PCRE1 is now in maintenance-only status, it is unlikely ever
    to change.

  For compatibility with Perl, assertion subpatterns can be
  repeated. However, it makes no sense to assert the same thing many
  times, the side effect of capturing parentheses can occasionally
  be useful. In practice, there are only three cases:

   • If the quantifier is {0}, the assertion is never obeyed
     during matching. However, it can contain internal capturing
     parenthesized groups that are called from elsewhere through
     the subroutine mechanism.

   • If quantifier is {0,n}, where n > 0, it is treated as if it
     was {0,1}. At runtime, the remaining pattern match is tried
     with and without the assertion, the order depends on the
     greediness of the quantifier.

   • If the minimum repetition is > 0, the quantifier is ignored.
     The assertion is obeyed only once when encountered during
     matching.

  Lookahead Assertions

  Lookahead assertions start with (?= for positive assertions and
  (?! for negative assertions. For example, the following matches a
  word followed by a semicolon, but does not include the semicolon
  in the match:

    \w+(?=;)

  The following matches any occurrence of "foo" that is not followed
  by "bar":

    foo(?!bar)

  Notice that the apparently similar pattern

    (?!foo)bar

  does not find an occurrence of "bar" that is preceded by something
  other than "foo". It finds any occurrence of "bar" whatsoever, as
  the assertion (?!foo) is always true when the next three
  characters are "bar". A lookbehind assertion is needed to achieve
  the other effect.

  If you want to force a matching failure at some point in a
  pattern, the most convenient way to do it is with (?!), as an
  empty string always matches. So, an assertion that requires there
  is not to be an empty string must always fail. The backtracking
  control verb (*FAIL) or (*F) is a synonym for (?!).

  Lookbehind Assertions

  Lookbehind assertions start with (?<= for positive assertions and
  (?<! for negative assertions. For example, the following finds an
  occurrence of "bar" that is not preceded by "foo":

    (?<!foo)bar

  The contents of a lookbehind assertion are restricted such that
  all the strings it matches must have a fixed length. However, if
  there are many top-level alternatives, they do not all have to
  have the same fixed length. Thus, the following is permitted:

    (?<=bullock|donkey)

  The following causes an error at compile time:

    (?<!dogs?|cats?)

  Branches that match different length strings are permitted only at
  the top-level of a lookbehind assertion. This is an extension
  compared with Perl, which requires all branches to match the same
  length of string. An assertion such as the following is not
  permitted, as its single top-level branch can match two different
  lengths:

    (?<=ab(c|de))

  However, it is acceptable to PCRE if rewritten to use two
  top-level branches:

    (?<=abc|abde)

  Sometimes the escape sequence \K (see above) can be used instead
  of a lookbehind assertion to get round the fixed-length
  restriction.

  The implementation of lookbehind assertions is, for each
  alternative, to move the current position back temporarily by the
  fixed length and then try to match. If there are insufficient
  characters before the current position, the assertion fails.

  In a UTF mode, PCRE does not allow the \C escape (which matches a
  single data unit even in a UTF mode) to appear in lookbehind
  assertions, as it makes it impossible to calculate the length of
  the lookbehind. The \X and \R escapes, which can match different
  numbers of data units, are not permitted either.

  "Subroutine" calls (see below), such as (?2) or (?&X), are
  permitted in lookbehinds, as long as the subpattern matches a
  fixed-length string. Recursion, however, is not supported.

  Possessive quantifiers can be used with lookbehind assertions to
  specify efficient matching of fixed-length strings at the end of
  subject strings. Consider the following simple pattern when
  applied to a long string that does not match:

    abcd$

  As matching proceeds from left to right, PCRE looks for each "a"
  in the subject and then sees if what follows matches the remaining
  pattern. If the pattern is specified as

    ^.*abcd$

  the initial .* matches the entire string at first. However, when
  this fails (as there is no following "a"), it backtracks to match
  all but the last character, then all but the last two characters,
  and so on. Once again the search for "a" covers the entire string,
  from right to left, so we are no better off. However, if the
  pattern is written as

    ^.*+(?<=abcd)

  there can be no backtracking for the .*+ item; it can match only
  the entire string. The subsequent lookbehind assertion does a
  single test on the last four characters. If it fails, the match
  fails immediately. For long strings, this approach makes a
  significant difference to the processing time.

  Using Multiple Assertions

  Many assertions (of any sort) can occur in succession. For
  example, the following matches "foo" preceded by three digits that
  are not "999":

    (?<=\d{3})(?<!999)foo

  Notice that each of the assertions is applied independently at the
  same point in the subject string. First there is a check that the
  previous three characters are all digits, and then there is a
  check that the same three characters are not "999". This pattern
  does not match "foo" preceded by six characters, the first of
  which are digits and the last three of which are not "999". For
  example, it does not match "123abcfoo". A pattern to do that is
  the following:

    (?<=\d{3}...)(?<!999)foo

  This time the first assertion looks at the preceding six
  characters, checks that the first three are digits, and then the
  second assertion checks that the preceding three characters are
  not "999".

  Assertions can be nested in any combination. For example, the
  following matches an occurrence of "baz" that is preceded by
  "bar", which in turn is not preceded by "foo":

    (?<=(?<!foo)bar)baz

  The following pattern matches "foo" preceded by three digits and
  any three characters that are not "999":

    (?<=\d{3}(?!999)...)foo

Conditional Subpatterns

  It is possible to cause the matching process to obey a subpattern
  conditionally or to choose between two alternative subpatterns,
  depending on the result of an assertion, or whether a specific
  capturing subpattern has already been matched. The following are
  the two possible forms of conditional subpattern:

    (?(condition)yes-pattern)
    (?(condition)yes-pattern|no-pattern)

  If the condition is satisfied, the yes-pattern is used, otherwise
  the no-pattern (if present). If more than two alternatives exist
  in the subpattern, a compile-time error occurs. Each of the two
  alternatives can itself contain nested subpatterns of any form,
  including conditional subpatterns; the restriction to two
  alternatives applies only at the level of the condition. The
  following pattern fragment is an example where the alternatives
  are complex:

    (?(1) (A|B|C) | (D | (?(2)E|F) | E) )

  There are four kinds of condition: references to subpatterns,
  references to recursion, a pseudo-condition called DEFINE, and
  assertions.

  Checking for a Used Subpattern By Number

  If the text between the parentheses consists of a sequence of
  digits, the condition is true if a capturing subpattern of that
  number has previously matched. If more than one capturing
  subpattern with the same number exists (see section Duplicate
  Subpattern Numbers earlier), the condition is true if any of them
  have matched. An alternative notation is to precede the digits
  with a plus or minus sign. In this case, the subpattern number is
  relative rather than absolute. The most recently opened
  parentheses can be referenced by (?(-1), the next most recent by
  (?(-2), and so on. Inside loops, it can also make sense to refer
  to subsequent groups. The next parentheses to be opened can be
  referenced as (?(+1), and so on. (The value zero in any of these
  forms is not used; it provokes a compile-time error.)

  Consider the following pattern, which contains non-significant
  whitespace to make it more readable (assume option extended) and
  to divide it into three parts for ease of discussion:

    ( \( )?    [^()]+    (?(1) \) )

  The first part matches an optional opening parenthesis, and if
  that character is present, sets it as the first captured
  substring. The second part matches one or more characters that are
  not parentheses. The third part is a conditional subpattern that
  tests whether the first set of parentheses matched or not. If they
  did, that is, if subject started with an opening parenthesis, the
  condition is true, and so the yes-pattern is executed and a
  closing parenthesis is required. Otherwise, as no-pattern is not
  present, the subpattern matches nothing. That is, this pattern
  matches a sequence of non-parentheses, optionally enclosed in
  parentheses.

  If this pattern is embedded in a larger one, a relative reference
  can be used:

    ...other stuff... ( \( )?    [^()]+    (?(-1) \) ) ...

  This makes the fragment independent of the parentheses in the
  larger pattern.

  Checking for a Used Subpattern By Name

  Perl uses the syntax (?(<name>)...) or (?('name')...) to test for
  a used subpattern by name. For compatibility with earlier versions
  of PCRE, which had this facility before Perl, the syntax
  (?(name)...) is also recognized.

  Rewriting the previous example to use a named subpattern gives:

    (?<OPEN> \( )?    [^()]+    (?(<OPEN>) \) )

  If the name used in a condition of this kind is a duplicate, the
  test is applied to all subpatterns of the same name, and is true
  if any one of them has matched.

  Checking for Pattern Recursion

  If the condition is the string (R), and there is no subpattern
  with the name R, the condition is true if a recursive call to the
  whole pattern or any subpattern has been made. If digits or a name
  preceded by ampersand follow the letter R, for example:

    (?(R3)...) or (?(R&name)...)

  the condition is true if the most recent recursion is into a
  subpattern whose number or name is given. This condition does not
  check the entire recursion stack. If the name used in a condition
  of this kind is a duplicate, the test is applied to all
  subpatterns of the same name, and is true if any one of them is
  the most recent recursion.

  At "top-level", all these recursion test conditions are false. The
  syntax for recursive patterns is described below.

  Defining Subpatterns for Use By Reference Only

  If the condition is the string (DEFINE), and there is no
  subpattern with the name DEFINE, the condition is always false. In
  this case, there can be only one alternative in the subpattern. It
  is always skipped if control reaches this point in the pattern.
  The idea of DEFINE is that it can be used to define "subroutines"
  that can be referenced from elsewhere. (The use of subroutines is
  described below.) For example, a pattern to match an IPv4 address,
  such as "192.168.23.245", can be written like this (ignore
  whitespace and line breaks):

    (?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) ) \b (?&byte) (\.(?&byte)){3} \b

  The first part of the pattern is a DEFINE group inside which is a
  another group named "byte" is defined. This matches an individual
  component of an IPv4 address (a number < 256). When matching takes
  place, this part of the pattern is skipped, as DEFINE acts like a
  false condition. The remaining pattern uses references to the
  named group to match the four dot-separated components of an IPv4
  address, insisting on a word boundary at each end.

  Assertion Conditions

  If the condition is not in any of the above formats, it must be an
  assertion. This can be a positive or negative lookahead or
  lookbehind assertion. Consider the following pattern, containing
  non-significant whitespace, and with the two alternatives on the
  second line:

    (?(?=[^a-z]*[a-z])
    \d{2}-[a-z]{3}-\d{2}  |  \d{2}-\d{2}-\d{2} )

  The condition is a positive lookahead assertion that matches an
  optional sequence of non-letters followed by a letter. That is, it
  tests for the presence of at least one letter in the subject. If a
  letter is found, the subject is matched against the first
  alternative, otherwise it is matched against the second. This
  pattern matches strings in one of the two forms dd-aaa-dd or
  dd-dd-dd, where aaa are letters and dd are digits.

Comments

  There are two ways to include comments in patterns that are
  processed by PCRE. In both cases, the start of the comment must
  not be in a character class, or in the middle of any other
  sequence of related characters such as (?: or a subpattern name or
  number. The characters that make up a comment play no part in the
  pattern matching.

  The sequence (?# marks the start of a comment that continues up to
  the next closing parenthesis. Nested parentheses are not
  permitted. If option PCRE_EXTENDED is set, an unescaped #
  character also introduces a comment, which in this case continues
  to immediately after the next newline character or character
  sequence in the pattern. Which characters are interpreted as
  newlines is controlled by the options passed to a compiling
  function or by a special sequence at the start of the pattern, as
  described in section Newline Conventions earlier.

  Notice that the end of this type of comment is a literal newline
  sequence in the pattern; escape sequences that happen to represent
  a newline do not count. For example, consider the following
  pattern when extended is set, and the default newline convention
  is in force:

    abc #comment \n still comment

  On encountering character #, pcre_compile() skips along, looking
  for a newline in the pattern. The sequence \n is still literal at
  this stage, so it does not terminate the comment. Only a character
  with code value 0x0a (the default newline) does so.

Recursive Patterns

  Consider the problem of matching a string in parentheses, allowing
  for unlimited nested parentheses. Without the use of recursion,
  the best that can be done is to use a pattern that matches up to
  some fixed depth of nesting. It is not possible to handle an
  arbitrary nesting depth.

  For some time, Perl has provided a facility that allows regular
  expressions to recurse (among other things). It does this by
  interpolating Perl code in the expression at runtime, and the code
  can refer to the expression itself. A Perl pattern using code
  interpolation to solve the parentheses problem can be created like
  this:

    $re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x;

  Item (?p{...}) interpolates Perl code at runtime, and in this case
  refers recursively to the pattern in which it appears.

  Obviously, PCRE cannot support the interpolation of Perl code.
  Instead, it supports special syntax for recursion of the entire
  pattern, and for individual subpattern recursion. After its
  introduction in PCRE and Python, this kind of recursion was later
  introduced into Perl at release 5.10.

  A special item that consists of (? followed by a number > 0 and a
  closing parenthesis is a recursive subroutine call of the
  subpattern of the given number, if it occurs inside that
  subpattern. (If not, it is a non-recursive subroutine call, which
  is described in the next section.) The special item (?R) or (?0)
  is a recursive call of the entire regular expression.

  This PCRE pattern solves the nested parentheses problem (assume
  that option extended is set so that whitespace is ignored):

    \( ( [^()]++ | (?R) )* \)

  First it matches an opening parenthesis. Then it matches any
  number of substrings, which can either be a sequence of
  non-parentheses or a recursive match of the pattern itself (that
  is, a correctly parenthesized substring). Finally there is a
  closing parenthesis. Notice the use of a possessive quantifier to
  avoid backtracking into sequences of non-parentheses.

  If this was part of a larger pattern, you would not want to
  recurse the entire pattern, so instead you can use:

    ( \( ( [^()]++ | (?1) )* \) )

  The pattern is here within parentheses so that the recursion
  refers to them instead of the whole pattern.

  In a larger pattern, keeping track of parenthesis numbers can be
  tricky. This is made easier by the use of relative references.
  Instead of (?1) in the pattern above, you can write (?-2) to refer
  to the second most recently opened parentheses preceding the
  recursion. That is, a negative number counts capturing parentheses
  leftwards from the point at which it is encountered.

  It is also possible to refer to later opened parentheses, by
  writing references such as (?+2). However, these cannot be
  recursive, as the reference is not inside the parentheses that are
  referenced. They are always non-recursive subroutine calls, as
  described in the next section.

  An alternative approach is to use named parentheses instead. The
  Perl syntax for this is (?&name). The earlier PCRE syntax
  (?P>name) is also supported. We can rewrite the above example as
  follows:

    (?<pn> \( ( [^()]++ | (?&pn) )* \) )

  If there is more than one subpattern with the same name, the
  earliest one is used.

  This particular example pattern that we have studied contains
  nested unlimited repeats, and so the use of a possessive
  quantifier for matching strings of non-parentheses is important
  when applying the pattern to strings that do not match. For
  example, when this pattern is applied to

    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()

  it gives "no match" quickly. However, if a possessive quantifier
  is not used, the match runs for a long time, as there are so many
  different ways the + and * repeats can carve up the subject, and
  all must be tested before failure can be reported.

  At the end of a match, the values of capturing parentheses are
  those from the outermost level. If the pattern above is matched
  against

    (ab(cd)ef)

  the value for the inner capturing parentheses (numbered 2) is
  "ef", which is the last value taken on at the top-level. If a
  capturing subpattern is not matched at the top level, its final
  captured value is unset, even if it was (temporarily) set at a
  deeper level during the matching process.

  Do not confuse item (?R) with condition (R), which tests for
  recursion. Consider the following pattern, which matches text in
  angle brackets, allowing for arbitrary nesting. Only digits are
  allowed in nested brackets (that is, when recursing), while any
  characters are permitted at the outer level.

    < (?: (?(R) \d++  | [^<>]*+) | (?R)) * >

  Here (?(R) is the start of a conditional subpattern, with two
  different alternatives for the recursive and non-recursive cases.
  Item (?R) is the actual recursive call.

  Differences in Recursion Processing between PCRE and Perl

  Recursion processing in PCRE differs from Perl in two important
  ways. In PCRE (like Python, but unlike Perl), a recursive
  subpattern call is always treated as an atomic group. That is,
  once it has matched some of the subject string, it is never
  re-entered, even if it contains untried alternatives and there is
  a subsequent matching failure. This can be illustrated by the
  following pattern, which means to match a palindromic string
  containing an odd number of characters (for example, "a", "aba",
  "abcba", "abcdcba"):

    ^(.|(.)(?1)\2)$

  The idea is that it either matches a single character, or two
  identical characters surrounding a subpalindrome. In Perl, this
  pattern works; in PCRE it does not work if the pattern is longer
  than three characters. Consider the subject string "abcba".

  At the top level, the first character is matched, but as it is not
  at the end of the string, the first alternative fails, the second
  alternative is taken, and the recursion kicks in. The recursive
  call to subpattern 1 successfully matches the next character
  ("b"). (Notice that the beginning and end of line tests are not
  part of the recursion.)

  Back at the top level, the next character ("c") is compared with
  what subpattern 2 matched, which was "a". This fails. As the
  recursion is treated as an atomic group, there are now no
  backtracking points, and so the entire match fails. (Perl can now
  re-enter the recursion and try the second alternative.) However,
  if the pattern is written with the alternatives in the other
  order, things are different:

    ^((.)(?1)\2|.)$

  This time, the recursing alternative is tried first, and continues
  to recurse until it runs out of characters, at which point the
  recursion fails. But this time we have another alternative to try
  at the higher level. That is the significant difference: in the
  previous case the remaining alternative is at a deeper recursion
  level, which PCRE cannot use.

  To change the pattern so that it matches all palindromic strings,
  not only those with an odd number of characters, it is tempting to
  change the pattern to this:

    ^((.)(?1)\2|.?)$

  Again, this works in Perl, but not in PCRE, and for the same
  reason. When a deeper recursion has matched a single character, it
  cannot be entered again to match an empty string. The solution is
  to separate the two cases, and write out the odd and even cases as
  alternatives at the higher level:

    ^(?:((.)(?1)\2|)|((.)(?3)\4|.))

  If you want to match typical palindromic phrases, the pattern must
  ignore all non-word characters, which can be done as follows:

    ^\W*+(?:((.)\W*+(?1)\W*+\2|)|((.)\W*+(?3)\W*+\4|\W*+.\W*+))\W*+$

  If run with option caseless, this pattern matches phrases such
  as "A man, a plan, a canal: Panama!" and it works well in both
  PCRE and Perl. Notice the use of the possessive quantifier *+ to
  avoid backtracking into sequences of non-word characters. Without
  this, PCRE takes much longer (10 times or more) to match typical
  phrases, and Perl takes so long that you think it has gone into a
  loop.

  Note:
    The palindrome-matching patterns above work only if the
    subject string does not start with a palindrome that is
    shorter than the entire string. For example, although "abcba"
    is correctly matched, if the subject is "ababa", PCRE finds
    palindrome "aba" at the start, and then fails at top level, as
    the end of the string does not follow. Once again, it cannot
    jump back into the recursion to try other alternatives, so the
    entire match fails.

  The second way in which PCRE and Perl differ in their recursion
  processing is in the handling of captured values. In Perl, when a
  subpattern is called recursively or as a subpattern (see the next
  section), it has no access to any values that were captured
  outside the recursion. In PCRE these values can be referenced.
  Consider the following pattern:

    ^(.)(\1|a(?2))

  In PCRE, it matches "bab". The first capturing parentheses match
  "b", then in the second group, when the back reference \1 fails to
  match "b", the second alternative matches "a", and then recurses.
  In the recursion, \1 does now match "b" and so the whole match
  succeeds. In Perl, the pattern fails to match because inside the
  recursive call \1 cannot access the externally set value.

Subpatterns as Subroutines

  If the syntax for a recursive subpattern call (either by number or
  by name) is used outside the parentheses to which it refers, it
  operates like a subroutine in a programming language. The called
  subpattern can be defined before or after the reference. A
  numbered reference can be absolute or relative, as in the
  following examples:

    (...(absolute)...)...(?2)...
    (...(relative)...)...(?-1)...
    (...(?+1)...(relative)...

  An earlier example pointed out that the following pattern matches
  "sense and sensibility" and "response and responsibility", but not
  "sense and responsibility":

    (sens|respons)e and \1ibility

  If instead the following pattern is used, it matches "sense and
  responsibility" and the other two strings:

    (sens|respons)e and (?1)ibility

  Another example is provided in the discussion of DEFINE earlier.

  All subroutine calls, recursive or not, are always treated as
  atomic groups. That is, once a subroutine has matched some of the
  subject string, it is never re-entered, even if it contains
  untried alternatives and there is a subsequent matching failure.
  Any capturing parentheses that are set during the subroutine call
  revert to their previous values afterwards.

  Processing options such as case-independence are fixed when a
  subpattern is defined, so if it is used as a subroutine, such
  options cannot be changed for different calls. For example, the
  following pattern matches "abcabc" but not "abcABC", as the change
  of processing option does not affect the called subpattern:

    (abc)(?i:(?-1))

Oniguruma Subroutine Syntax

  For compatibility with Oniguruma, the non-Perl syntax \g followed
  by a name or a number enclosed either in angle brackets or single
  quotes, is alternative syntax for referencing a subpattern as a
  subroutine, possibly recursively. Here follows two of the examples
  used above, rewritten using this syntax:

    (?<pn> \( ( (?>[^()]+) | \g<pn> )* \) )
    (sens|respons)e and \g'1'ibility

  PCRE supports an extension to Oniguruma: if a number is preceded
  by a plus or minus sign, it is taken as a relative reference, for
  example:

    (abc)(?i:\g<-1>)

  Notice that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax)
  are not synonymous. The former is a back reference; the latter
  is a subroutine call.

Backtracking Control

  Perl 5.10 introduced some "Special Backtracking Control Verbs",
  which are still described in the Perl documentation as
  "experimental and subject to change or removal in a future version
  of Perl". It goes on to say: "Their usage in production code
  should be noted to avoid problems during upgrades." The same
  remarks apply to the PCRE features described in this section.

  The new verbs make use of what was previously invalid syntax: an
  opening parenthesis followed by an asterisk. They are generally of
  the form (*VERB) or (*VERB:NAME). Some can take either form,
  possibly behaving differently depending on whether a name is
  present. A name is any sequence of characters that does not
  include a closing parenthesis. The maximum name length is 255 in
  the 8-bit library and 65535 in the 16-bit and 32-bit libraries. If
  the name is empty, that is, if the closing parenthesis immediately
  follows the colon, the effect is as if the colon was not there.
  Any number of these verbs can occur in a pattern.

  The behavior of these verbs in repeated groups, assertions, and in
  subpatterns called as subroutines (whether or not recursively) is
  described below.

  Optimizations That Affect Backtracking Verbs

  PCRE contains some optimizations that are used to speed up
  matching by running some checks at the start of each match
  attempt. For example, it can know the minimum length of matching
  subject, or that a particular character must be present. When one
  of these optimizations bypasses the running of a match, any
  included backtracking verbs are not processed. processed. You can
  suppress the start-of-match optimizations by setting option 
  no_start_optimize when calling compile/2 or run/3, or by
  starting the pattern with (*NO_START_OPT).

  Experiments with Perl suggest that it too has similar
  optimizations, sometimes leading to anomalous results.

  Verbs That Act Immediately

  The following verbs act as soon as they are encountered. They must
  not be followed by a name.

    (*ACCEPT)

  This verb causes the match to end successfully, skipping the
  remainder of the pattern. However, when it is inside a subpattern
  that is called as a subroutine, only that subpattern is ended
  successfully. Matching then continues at the outer level. If
  (*ACCEPT) is triggered in a positive assertion, the assertion
  succeeds; in a negative assertion, the assertion fails.

  If (*ACCEPT) is inside capturing parentheses, the data so far is
  captured. For example, the following matches "AB", "AAD", or
  "ACD". When it matches "AB", "B" is captured by the outer
  parentheses.

    A((?:A|B(*ACCEPT)|C)D)

  The following verb causes a matching failure, forcing backtracking
  to occur. It is equivalent to (?!) but easier to read.

    (*FAIL) or (*F)

  The Perl documentation states that it is probably useful only when
  combined with (?{}) or (??{}). Those are Perl features that are
  not present in PCRE.

  A match with the string "aaaa" always fails, but the callout is
  taken before each backtrack occurs (in this example, 10 times).

  Recording Which Path Was Taken

  The main purpose of this verb is to track how a match was arrived
  at, although it also has a secondary use in with advancing the
  match starting point (see (*SKIP) below).

  Note:
    In Erlang, there is no interface to retrieve a mark with 
    run/2,3, so only the secondary purpose is relevant to the
    Erlang programmer.

    The rest of this section is therefore deliberately not adapted
    for reading by the Erlang programmer, but the examples can
    help in understanding NAMES as they can be used by (*SKIP).

    (*MARK:NAME) or (*:NAME)

  A name is always required with this verb. There can be as many
  instances of (*MARK) as you like in a pattern, and their names do
  not have to be unique.

  When a match succeeds, the name of the last encountered
  (*MARK:NAME), (*PRUNE:NAME), or (*THEN:NAME) on the matching path
  is passed back to the caller as described in section "Extra data
  for pcre_exec()" in the pcreapi documentation. In the
  following example of pcretest output, the /K modifier requests
  the retrieval and outputting of (*MARK) data:

      re> /X(*MARK:A)Y|X(*MARK:B)Z/K
    data> XY
     0: XY
    MK: A
    XZ
     0: XZ
    MK: B

  The (*MARK) name is tagged with "MK:" in this output, and in this
  example it indicates which of the two alternatives matched. This
  is a more efficient way of obtaining this information than putting
  each alternative in its own capturing parentheses.

  If a verb with a name is encountered in a positive assertion that
  is true, the name is recorded and passed back if it is the last
  encountered. This does not occur for negative assertions or
  failing positive assertions.

  After a partial match or a failed match, the last encountered name
  in the entire match process is returned, for example:

      re> /X(*MARK:A)Y|X(*MARK:B)Z/K
    data> XP
    No match, mark = B

  Notice that in this unanchored example, the mark is retained from
  the match attempt that started at letter "X" in the subject.
  Subsequent match attempts starting at "P" and then with an empty
  string do not get as far as the (*MARK) item, nevertheless do not
  reset it.

  Verbs That Act after Backtracking

  The following verbs do nothing when they are encountered. Matching
  continues with what follows, but if there is no subsequent match,
  causing a backtrack to the verb, a failure is forced. That is,
  backtracking cannot pass to the left of the verb. However, when
  one of these verbs appears inside an atomic group or an assertion
  that is true, its effect is confined to that group, as once the
  group has been matched, there is never any backtracking into it.
  In this situation, backtracking can "jump back" to the left of the
  entire atomic group or assertion. (Remember also, as stated above,
  that this localization also applies in subroutine calls.)

  These verbs differ in exactly what kind of failure occurs when
  backtracking reaches them. The behavior described below is what
  occurs when the verb is not in a subroutine or an assertion.
  Subsequent sections cover these special cases.

  The following verb, which must not be followed by a name, causes
  the whole match to fail outright if there is a later matching
  failure that causes backtracking to reach it. Even if the pattern
  is unanchored, no further attempts to find a match by advancing
  the starting point take place.

    (*COMMIT)

  If (*COMMIT) is the only backtracking verb that is encountered,
  once it has been passed, run/2,3 is committed to find a match at
  the current starting point, or not at all, for example:

    a+(*COMMIT)b

  This matches "xxaab" but not "aacaab". It can be thought of as a
  kind of dynamic anchor, or "I've started, so I must finish". The
  name of the most recently passed (*MARK) in the path is passed
  back when (*COMMIT) forces a match failure.

  If more than one backtracking verb exists in a pattern, a
  different one that follows (*COMMIT) can be triggered first, so
  merely passing (*COMMIT) during a match does not always guarantee
  that a match must be at this starting point.

  Notice that (*COMMIT) at the start of a pattern is not the same as
  an anchor, unless the PCRE start-of-match optimizations are turned
  off, as shown in the following example:

    1> re:run("xyzabc","(*COMMIT)abc",[{capture,all,list}]).
    {match,["abc"]}
    2> re:run("xyzabc","(*COMMIT)abc",[{capture,all,list},no_start_optimize]).
    nomatch

  For this pattern, PCRE knows that any match must start with "a",
  so the optimization skips along the subject to "a" before applying
  the pattern to the first set of data. The match attempt then
  succeeds. In the second call the no_start_optimize disables the
  optimization that skips along to the first character. The pattern
  is now applied starting at "x", and so the (*COMMIT) causes the
  match to fail without trying any other starting points.

  The following verb causes the match to fail at the current
  starting position in the subject if there is a later matching
  failure that causes backtracking to reach it:

    (*PRUNE) or (*PRUNE:NAME)

  If the pattern is unanchored, the normal "bumpalong" advance to
  the next starting character then occurs. Backtracking can occur as
  usual to the left of (*PRUNE), before it is reached, or when
  matching to the right of (*PRUNE), but if there is no match to the
  right, backtracking cannot cross (*PRUNE). In simple cases, the
  use of (*PRUNE) is just an alternative to an atomic group or
  possessive quantifier, but there are some uses of (*PRUNE) that
  cannot be expressed in any other way. In an anchored pattern,
  (*PRUNE) has the same effect as (*COMMIT).

  The behavior of (*PRUNE:NAME) is the not the same as
  (*MARK:NAME)(*PRUNE). It is like (*MARK:NAME) in that the name is
  remembered for passing back to the caller. However, (*SKIP:NAME)
  searches only for names set with (*MARK).

  Note:
    The fact that (*PRUNE:NAME) remembers the name is useless to
    the Erlang programmer, as names cannot be retrieved.

  The following verb, when specified without a name, is like
  (*PRUNE), except that if the pattern is unanchored, the
  "bumpalong" advance is not to the next character, but to the
  position in the subject where (*SKIP) was encountered.

    (*SKIP)

  (*SKIP) signifies that whatever text was matched leading up to it
  cannot be part of a successful match. Consider:

    a+(*SKIP)b

  If the subject is "aaaac...", after the first match attempt fails
  (starting at the first character in the string), the starting
  point skips on to start the next attempt at "c". Notice that a
  possessive quantifier does not have the same effect as this
  example; although it would suppress backtracking during the first
  match attempt, the second attempt would start at the second
  character instead of skipping on to "c".

  When (*SKIP) has an associated name, its behavior is modified:

    (*SKIP:NAME)

  When this is triggered, the previous path through the pattern is
  searched for the most recent (*MARK) that has the same name. If
  one is found, the "bumpalong" advance is to the subject position
  that corresponds to that (*MARK) instead of to where (*SKIP) was
  encountered. If no (*MARK) with a matching name is found, (*SKIP)
  is ignored.

  Notice that (*SKIP:NAME) searches only for names set by
  (*MARK:NAME). It ignores names that are set by (*PRUNE:NAME) or
  (*THEN:NAME).

  The following verb causes a skip to the next innermost alternative
  when backtracking reaches it. That is, it cancels any further
  backtracking within the current alternative.

    (*THEN) or (*THEN:NAME)

  The verb name comes from the observation that it can be used for a
  pattern-based if-then-else block:

    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...

  If the COND1 pattern matches, FOO is tried (and possibly further
  items after the end of the group if FOO succeeds). On failure, the
  matcher skips to the second alternative and tries COND2, without
  backtracking into COND1. If that succeeds and BAR fails, COND3 is
  tried. If BAZ then fails, there are no more alternatives, so there
  is a backtrack to whatever came before the entire group. If
  (*THEN) is not inside an alternation, it acts like (*PRUNE).

  The behavior of (*THEN:NAME) is the not the same as
  (*MARK:NAME)(*THEN). It is like (*MARK:NAME) in that the name is
  remembered for passing back to the caller. However, (*SKIP:NAME)
  searches only for names set with (*MARK).

  Note:
    The fact that (*THEN:NAME) remembers the name is useless to
    the Erlang programmer, as names cannot be retrieved.

  A subpattern that does not contain a | character is just a part of
  the enclosing alternative; it is not a nested alternation with
  only one alternative. The effect of (*THEN) extends beyond such a
  subpattern to the enclosing alternative. Consider the following
  pattern, where A, B, and so on, are complex pattern fragments that
  do not contain any | characters at this level:

    A (B(*THEN)C) | D

  If A and B are matched, but there is a failure in C, matching does
  not backtrack into A; instead it moves to the next alternative,
  that is, D. However, if the subpattern containing (*THEN) is given
  an alternative, it behaves differently:

    A (B(*THEN)C | (*FAIL)) | D

  The effect of (*THEN) is now confined to the inner subpattern.
  After a failure in C, matching moves to (*FAIL), which causes the
  whole subpattern to fail, as there are no more alternatives to
  try. In this case, matching does now backtrack into A.

  Notice that a conditional subpattern is not considered as having
  two alternatives, as only one is ever used. That is, the |
  character in a conditional subpattern has a different meaning.
  Ignoring whitespace, consider:

    ^.*? (?(?=a) a | b(*THEN)c )

  If the subject is "ba", this pattern does not match. As .*? is
  ungreedy, it initially matches zero characters. The condition
  (?=a) then fails, the character "b" is matched, but "c" is not. At
  this point, matching does not backtrack to .*? as can perhaps be
  expected from the presence of the | character. The conditional
  subpattern is part of the single alternative that comprises the
  whole pattern, and so the match fails. (If there was a backtrack
  into .*?, allowing it to match "b", the match would succeed.)

  The verbs described above provide four different "strengths" of
  control when subsequent matching fails:

   • (*THEN) is the weakest, carrying on the match at the next
     alternative.

   • (*PRUNE) comes next, fails the match at the current starting
     position, but allows an advance to the next character (for
     an unanchored pattern).

   • (*SKIP) is similar, except that the advance can be more than
     one character.

   • (*COMMIT) is the strongest, causing the entire match to
     fail.

  More than One Backtracking Verb

  If more than one backtracking verb is present in a pattern, the
  one that is backtracked onto first acts. For example, consider the
  following pattern, where A, B, and so on, are complex pattern
  fragments:

    (A(*COMMIT)B(*THEN)C|ABD)

  If A matches but B fails, the backtrack to (*COMMIT) causes the
  entire match to fail. However, if A and B match, but C fails, the
  backtrack to (*THEN) causes the next alternative (ABD) to be
  tried. This behavior is consistent, but is not always the same as
  in Perl. It means that if two or more backtracking verbs appear in
  succession, the last of them has no effect. Consider the following
  example:

    ...(*COMMIT)(*PRUNE)...

  If there is a matching failure to the right, backtracking onto
  (*PRUNE) causes it to be triggered, and its action is taken. There
  can never be a backtrack onto (*COMMIT).

  Backtracking Verbs in Repeated Groups

  PCRE differs from Perl in its handling of backtracking verbs in
  repeated groups. For example, consider:

    /(a(*COMMIT)b)+ac/

  If the subject is "abac", Perl matches, but PCRE fails because the
  (*COMMIT) in the second repeat of the group acts.

  Backtracking Verbs in Assertions

  (*FAIL) in an assertion has its normal effect: it forces an
  immediate backtrack.

  (*ACCEPT) in a positive assertion causes the assertion to succeed
  without any further processing. In a negative assertion, (*ACCEPT)
  causes the assertion to fail without any further processing.

  The other backtracking verbs are not treated specially if they
  appear in a positive assertion. In particular, (*THEN) skips to
  the next alternative in the innermost enclosing group that has
  alternations, regardless if this is within the assertion.

  Negative assertions are, however, different, to ensure that
  changing a positive assertion into a negative assertion changes
  its result. Backtracking into (*COMMIT), (*SKIP), or (*PRUNE)
  causes a negative assertion to be true, without considering any
  further alternative branches in the assertion. Backtracking into
  (*THEN) causes it to skip to the next enclosing alternative within
  the assertion (the normal behavior), but if the assertion does not
  have such an alternative, (*THEN) behaves like (*PRUNE).

  Backtracking Verbs in Subroutines

  These behaviors occur regardless if the subpattern is called
  recursively. The treatment of subroutines in Perl is different in
  some cases.

   • (*FAIL) in a subpattern called as a subroutine has its
     normal effect: it forces an immediate backtrack.

   • (*ACCEPT) in a subpattern called as a subroutine causes the
     subroutine match to succeed without any further processing.
     Matching then continues after the subroutine call.

   • (*COMMIT), (*SKIP), and (*PRUNE) in a subpattern called as a
     subroutine cause the subroutine match to fail.

   • (*THEN) skips to the next alternative in the innermost
     enclosing group within the subpattern that has alternatives.
     If there is no such group within the subpattern, (*THEN)
     causes the subroutine match to fail.