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<title>Regex++, Regular Expression Syntax</title>
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<p> </p>
<table border="0" cellpadding="7" cellspacing="0" width="100%">
<tr>
<td valign="top"><h3><img src="../../c++boost.gif"
alt="C++ Boost" width="276" height="86"></h3>
</td>
<td valign="top"><h3 align="center">Regex++, Regular
Expression Syntax.</h3>
<p align="left"><i>Copyright (c) 1998-2001 </i></p>
<p align="left"><i>Dr John Maddock</i></p>
<p align="left"><i>Permission to use, copy, modify,
distribute and sell this software and its documentation
for any purpose is hereby granted without fee, provided
that the above copyright notice appear in all copies and
that both that copyright notice and this permission
notice appear in supporting documentation. Dr John
Maddock makes no representations about the suitability of
this software for any purpose. It is provided "as is"
without express or implied warranty.</i></p>
</td>
</tr>
</table>
<hr>
<h3><a name="syntax"></a><i>Regular expression syntax</i></h3>
<p>This section covers the regular expression syntax used by this
library, this is a programmers guide, the actual syntax presented
to your program's users will depend upon the flags used during
expression compilation. </p>
<p><i>Literals</i> </p>
<p>All characters are literals except: ".",
"*", "?", "+", "(",
")", "{", "}", "[",
"]", "^", "$" and "\".
These characters are literals when preceded by a "\". A
literal is a character that matches itself, or matches the result
of traits_type::translate(), where traits_type is the traits
template parameter to class reg_expression. <br>
<br>
</p>
<p><i>Wildcard</i> </p>
<p>The dot character "." matches any single character
except : when <i>match_not_dot_null</i> is passed to the matching
algorithms, the dot does not match a null character; when <i>match_not_dot_newline</i>
is passed to the matching algorithms, then the dot does not match
a newline character. <br>
<br>
</p>
<p><i>Repeats</i> </p>
<p>A repeat is an expression that is repeated an arbitrary number
of times. An expression followed by "*" can be repeated
any number of times including zero. An expression followed by
"+" can be repeated any number of times, but at least
once, if the expression is compiled with the flag regbase::bk_plus_qm
then "+" is an ordinary character and "\+"
represents a repeat of once or more. An expression followed by
"?" may be repeated zero or one times only, if the
expression is compiled with the flag regbase::bk_plus_qm then
"?" is an ordinary character and "\?"
represents the repeat zero or once operator. When it is necessary
to specify the minimum and maximum number of repeats explicitly,
the bounds operator "{}" may be used, thus "a{2}"
is the letter "a" repeated exactly twice, "a{2,4}"
represents the letter "a" repeated between 2 and 4
times, and "a{2,}" represents the letter "a"
repeated at least twice with no upper limit. Note that there must
be no white-space inside the {}, and there is no upper limit on
the values of the lower and upper bounds. When the expression is
compiled with the flag regbase::bk_braces then "{" and
"}" are ordinary characters and "\{" and
"\}" are used to delimit bounds instead. All repeat
expressions refer to the shortest possible previous sub-expression:
a single character; a character set, or a sub-expression grouped
with "()" for example. </p>
<p>Examples: </p>
<p>"ba*" will match all of "b", "ba",
"baaa" etc. </p>
<p>"ba+" will match "ba" or "baaaa"
for example but not "b". </p>
<p>"ba?" will match "b" or "ba". </p>
<p>"ba{2,4}" will match "baa", "baaa"
and "baaaa". </p>
<p><i>Non-greedy repeats</i> </p>
<p>Whenever the "extended" regular expression syntax is
in use (the default) then non-greedy repeats are possible by
appending a '?' after the repeat; a non-greedy repeat is one
which will match the <i>shortest</i> possible string. </p>
<p>For example to match html tag pairs one could use something
like: </p>
<p>"<\s*tagname[^>]*>(.*?)<\s*/tagname\s*>"
</p>
<p>In this case $1 will contain the text between the tag pairs,
and will be the shortest possible matching string. <br>
<br>
</p>
<p><i>Parenthesis</i> </p>
<p>Parentheses serve two purposes, to group items together into a
sub-expression, and to mark what generated the match. For example
the expression "(ab)*" would match all of the string
"ababab". The matching algorithms <a
href="template_class_ref.htm#query_match">regex_match</a> and <a
href="template_class_ref.htm#reg_search">regex_search</a> each
take an instance of <a
href="template_class_ref.htm#match_results">match_results</a>
that reports what caused the match, on exit from these functions
the <a href="template_class_ref.htm#match_results">match_results</a>
contains information both on what the whole expression matched
and on what each sub-expression matched. In the example above
match_results[1] would contain a pair of iterators denoting the
final "ab" of the matching string. It is permissible
for sub-expressions to match null strings. If a sub-expression
takes no part in a match - for example if it is part of an
alternative that is not taken - then both of the iterators that
are returned for that sub-expression point to the end of the
input string, and the <i>matched</i> parameter for that sub-expression
is <i>false</i>. Sub-expressions are indexed from left to right
starting from 1, sub-expression 0 is the whole expression. </p>
<p><i>Non-Marking Parenthesis</i> </p>
<p>Sometimes you need to group sub-expressions with parenthesis,
but don't want the parenthesis to spit out another marked sub-expression,
in this case a non-marking parenthesis (?:expression) can be used.
For example the following expression creates no sub-expressions: </p>
<p>"(?:abc)*" <br>
<br>
</p>
<p><i>Alternatives</i> </p>
<p>Alternatives occur when the expression can match either one
sub-expression or another, each alternative is separated by a
"|", or a "\|" if the flag regbase::bk_vbar
is set, or by a newline character if the flag regbase::newline_alt
is set. Each alternative is the largest possible previous sub-expression;
this is the opposite behaviour from repetition operators. </p>
<p>Examples: </p>
<p>"a(b|c)" could match "ab" or "ac".
</p>
<p>"abc|def" could match "abc" or "def".
<br>
<br>
</p>
<p><i>Sets</i> </p>
<p>A set is a set of characters that can match any single
character that is a member of the set. Sets are delimited by
"[" and "]" and can contain literals,
character ranges, character classes, collating elements and
equivalence classes. Set declarations that start with "^"
contain the compliment of the elements that follow. </p>
<p>Examples: </p>
<p>Character literals: </p>
<p>"[abc]" will match either of "a", "b",
or "c". </p>
<p>"[^abc] will match any character other than "a",
"b", or "c". </p>
<p>Character ranges: </p>
<p>"[a-z]" will match any character in the range "a"
to "z". </p>
<p>"[^A-Z]" will match any character other than those
in the range "A" to "Z". </p>
<p>Note that character ranges are highly locale dependent: they
match any character that collates between the endpoints of the
range, ranges will only behave according to ASCII rules when the
default "C" locale is in effect. For example if the
library is compiled with the Win32 localization model, then [a-z]
will match the ASCII characters a-z, and also 'A', 'B' etc, but
not 'Z' which collates just after 'z'. This locale specific
behaviour can be disabled by specifying regbase::nocollate when
compiling, this is the default behaviour when using regbase::normal,
and forces ranges to collate according to ASCII character code.
Likewise, if you use the POSIX C API functions then setting
REG_NOCOLLATE turns off locale dependent collation. </p>
<p>Character classes are denoted using the syntax "[:classname:]"
within a set declaration, for example "[[:space:]]" is
the set of all whitespace characters. Character classes are only
available if the flag regbase::char_classes is set. The available
character classes are: <br>
</p>
<table border="0" cellpadding="7" cellspacing="0" width="100%">
<tr>
<td width="5%"> </td>
<td valign="top" width="50%">alnum</td>
<td valign="top" width="50%">Any alpha numeric character.</td>
<td width="5%"> </td>
</tr>
<tr>
<td> </td>
<td valign="top" width="50%">alpha</td>
<td valign="top" width="50%">Any alphabetical character a-z
and A-Z. Other characters may also be included depending
upon the locale.</td>
<td> </td>
</tr>
<tr>
<td> </td>
<td valign="top" width="50%">blank</td>
<td valign="top" width="50%">Any blank character, either
a space or a tab.</td>
<td> </td>
</tr>
<tr>
<td> </td>
<td valign="top" width="50%">cntrl</td>
<td valign="top" width="50%">Any control character.</td>
<td> </td>
</tr>
<tr>
<td> </td>
<td valign="top" width="50%">digit</td>
<td valign="top" width="50%">Any digit 0-9.</td>
<td> </td>
</tr>
<tr>
<td> </td>
<td valign="top" width="50%">graph</td>
<td valign="top" width="50%">Any graphical character.</td>
<td> </td>
</tr>
<tr>
<td> </td>
<td valign="top" width="50%">lower</td>
<td valign="top" width="50%">Any lower case character a-z.
Other characters may also be included depending upon the
locale.</td>
<td> </td>
</tr>
<tr>
<td> </td>
<td valign="top" width="50%">print</td>
<td valign="top" width="50%">Any printable character.</td>
<td> </td>
</tr>
<tr>
<td> </td>
<td valign="top" width="50%">punct</td>
<td valign="top" width="50%">Any punctuation character.</td>
<td> </td>
</tr>
<tr>
<td> </td>
<td valign="top" width="50%">space</td>
<td valign="top" width="50%">Any whitespace character.</td>
<td> </td>
</tr>
<tr>
<td> </td>
<td valign="top" width="50%">upper</td>
<td valign="top" width="50%">Any upper case character A-Z.
Other characters may also be included depending upon the
locale.</td>
<td> </td>
</tr>
<tr>
<td> </td>
<td valign="top" width="50%">xdigit</td>
<td valign="top" width="50%">Any hexadecimal digit
character, 0-9, a-f and A-F.</td>
<td> </td>
</tr>
<tr>
<td> </td>
<td valign="top" width="50%">word</td>
<td valign="top" width="50%">Any word character - all
alphanumeric characters plus the underscore.</td>
<td> </td>
</tr>
<tr>
<td> </td>
<td valign="top" width="50%">unicode</td>
<td valign="top" width="50%">Any character whose code is
greater than 255, this applies to the wide character
traits classes only.</td>
<td> </td>
</tr>
</table>
<p>There are some shortcuts that can be used in place of the
character classes, provided the flag regbase::escape_in_lists is
set then you can use: </p>
<p>\w in place of [:word:] </p>
<p>\s in place of [:space:] </p>
<p>\d in place of [:digit:] </p>
<p>\l in place of [:lower:] </p>
<p>\u in place of [:upper:] <br>
<br>
</p>
<p>Collating elements take the general form [.tagname.] inside a
set declaration, where <i>tagname</i> is either a single
character, or a name of a collating element, for example [[.a.]]
is equivalent to [a], and [[.comma.]] is equivalent to [,]. The
library supports all the standard POSIX collating element names,
and in addition the following digraphs: "ae", "ch",
"ll", "ss", "nj", "dz",
"lj", each in lower, upper and title case variations.
Multi-character collating elements can result in the set matching
more than one character, for example [[.ae.]] would match two
characters, but note that [^[.ae.]] would only match one
character. <br>
<br>
</p>
<p>Equivalence classes take the general form [=tagname=] inside a
set declaration, where <i>tagname</i> is either a single
character, or a name of a collating element, and matches any
character that is a member of the same primary equivalence class
as the collating element [.tagname.]. An equivalence class is a
set of characters that collate the same, a primary equivalence
class is a set of characters whose primary sort key are all the
same (for example strings are typically collated by character,
then by accent, and then by case; the primary sort key then
relates to the character, the secondary to the accentation, and
the tertiary to the case). If there is no equivalence class
corresponding to <i>tagname</i>, then [=tagname=] is exactly the
same as [.tagname.]. Unfortunately there is no locale independent
method of obtaining the primary sort key for a character, except
under Win32. For other operating systems the library will "guess"
the primary sort key from the full sort key (obtained from <i>strxfrm</i>),
so equivalence classes are probably best considered broken under
any operating system other than Win32. <br>
<br>
</p>
<p>To include a literal "-" in a set declaration then:
make it the first character after the opening "[" or
"[^", the endpoint of a range, a collating element, or
if the flag regbase::escape_in_lists is set then precede with an
escape character as in "[\-]". To include a literal
"[" or "]" or "^" in a set then
make them the endpoint of a range, a collating element, or
precede with an escape character if the flag regbase::escape_in_lists
is set. <br>
<br>
</p>
<p><i>Line anchors</i> </p>
<p>An anchor is something that matches the null string at the
start or end of a line: "^" matches the null string at
the start of a line, "$" matches the null string at the
end of a line. <br>
<br>
</p>
<p><i>Back references</i> </p>
<p>A back reference is a reference to a previous sub-expression
that has already been matched, the reference is to what the sub-expression
matched, not to the expression itself. A back reference consists
of the escape character "\" followed by a digit "1"
to "9", "\1" refers to the first sub-expression,
"\2" to the second etc. For example the expression
"(.*)\1" matches any string that is repeated about its
mid-point for example "abcabc" or "xyzxyz". A
back reference to a sub-expression that did not participate in
any match, matches the null string: NB this is different to some
other regular expression matchers. Back references are only
available if the expression is compiled with the flag regbase::bk_refs
set. <br>
<br>
</p>
<p><i>Characters by code</i> </p>
<p>This is an extension to the algorithm that is not available in
other libraries, it consists of the escape character followed by
the digit "0" followed by the octal character code. For
example "\023" represents the character whose octal
code is 23. Where ambiguity could occur use parentheses to break
the expression up: "\0103" represents the character
whose code is 103, "(\010)3 represents the character 10
followed by "3". To match characters by their
hexadecimal code, use \x followed by a string of hexadecimal
digits, optionally enclosed inside {}, for example \xf0 or
\x{aff}, notice the latter example is a Unicode character. <br>
<br>
</p>
<p><i>Word operators</i> </p>
<p>The following operators are provided for compatibility with
the GNU regular expression library. </p>
<p>"\w" matches any single character that is a member
of the "word" character class, this is identical to the
expression "[[:word:]]". </p>
<p>"\W" matches any single character that is not a
member of the "word" character class, this is identical
to the expression "[^[:word:]]". </p>
<p>"\<" matches the null string at the start of a
word. </p>
<p>"\>" matches the null string at the end of the
word. </p>
<p>"\b" matches the null string at either the start or
the end of a word. </p>
<p>"\B" matches a null string within a word. </p>
<p>The start of the sequence passed to the matching algorithms is
considered to be a potential start of a word unless the flag
match_not_bow is set. The end of the sequence passed to the
matching algorithms is considered to be a potential end of a word
unless the flag match_not_eow is set. <br>
<br>
</p>
<p><i>Buffer operators</i> </p>
<p>The following operators are provide for compatibility with the
GNU regular expression library, and Perl regular expressions: </p>
<p>"\`" matches the start of a buffer. </p>
<p>"\A" matches the start of the buffer. </p>
<p>"\'" matches the end of a buffer. </p>
<p>"\z" matches the end of a buffer. </p>
<p>"\Z" matches the end of a buffer, or possibly one or
more new line characters followed by the end of the buffer. </p>
<p>A buffer is considered to consist of the whole sequence passed
to the matching algorithms, unless the flags match_not_bob or
match_not_eob are set. <br>
<br>
</p>
<p><i>Escape operator</i> </p>
<p>The escape character "\" has several meanings. </p>
<p>Inside a set declaration the escape character is a normal
character unless the flag regbase::escape_in_lists is set in
which case whatever follows the escape is a literal character
regardless of its normal meaning. </p>
<p>The escape operator may introduce an operator for example:
back references, or a word operator. </p>
<p>The escape operator may make the following character normal,
for example "\*" represents a literal "*"
rather than the repeat operator. <br>
<br>
</p>
<p><i>Single character escape sequences</i> </p>
<p>The following escape sequences are aliases for single
characters: <br>
</p>
<table border="0" cellpadding="7" cellspacing="0" width="100%">
<tr>
<td width="5%"> </td>
<td valign="top" width="33%">Escape sequence </td>
<td valign="top" width="33%">Character code </td>
<td valign="top" width="33%">Meaning </td>
<td width="5%"> </td>
</tr>
<tr>
<td> </td>
<td valign="top" width="33%">\a </td>
<td valign="top" width="33%">0x07 </td>
<td valign="top" width="33%">Bell character. </td>
<td> </td>
</tr>
<tr>
<td> </td>
<td valign="top" width="33%">\f </td>
<td valign="top" width="33%">0x0C </td>
<td valign="top" width="33%">Form feed. </td>
<td> </td>
</tr>
<tr>
<td> </td>
<td valign="top" width="33%">\n </td>
<td valign="top" width="33%">0x0A </td>
<td valign="top" width="33%">Newline character. </td>
<td> </td>
</tr>
<tr>
<td> </td>
<td valign="top" width="33%">\r </td>
<td valign="top" width="33%">0x0D </td>
<td valign="top" width="33%">Carriage return. </td>
<td> </td>
</tr>
<tr>
<td> </td>
<td valign="top" width="33%">\t </td>
<td valign="top" width="33%">0x09 </td>
<td valign="top" width="33%">Tab character. </td>
<td> </td>
</tr>
<tr>
<td> </td>
<td valign="top" width="33%">\v </td>
<td valign="top" width="33%">0x0B </td>
<td valign="top" width="33%">Vertical tab. </td>
<td> </td>
</tr>
<tr>
<td> </td>
<td valign="top" width="33%">\e </td>
<td valign="top" width="33%">0x1B </td>
<td valign="top" width="33%">ASCII Escape character. </td>
<td> </td>
</tr>
<tr>
<td> </td>
<td valign="top" width="33%">\0dd </td>
<td valign="top" width="33%">0dd </td>
<td valign="top" width="33%">An octal character code,
where <i>dd</i> is one or more octal digits. </td>
<td> </td>
</tr>
<tr>
<td> </td>
<td valign="top" width="33%">\xXX </td>
<td valign="top" width="33%">0xXX </td>
<td valign="top" width="33%">A hexadecimal character
code, where XX is one or more hexadecimal digits. </td>
<td> </td>
</tr>
<tr>
<td> </td>
<td valign="top" width="33%">\x{XX} </td>
<td valign="top" width="33%">0xXX </td>
<td valign="top" width="33%">A hexadecimal character
code, where XX is one or more hexadecimal digits,
optionally a unicode character. </td>
<td> </td>
</tr>
<tr>
<td> </td>
<td valign="top" width="33%">\cZ </td>
<td valign="top" width="33%">z-@ </td>
<td valign="top" width="33%">An ASCII escape sequence
control-Z, where Z is any ASCII character greater than or
equal to the character code for '@'. </td>
<td> </td>
</tr>
</table>
<p><br>
</p>
<p><i>Miscellaneous escape sequences:</i> </p>
<p>The following are provided mostly for perl compatibility, but
note that there are some differences in the meanings of \l \L \u
and \U: <br>
</p>
<table border="0" cellpadding="6" cellspacing="0" width="100%">
<tr>
<td width="5%"> </td>
<td valign="top" width="45%">\w </td>
<td valign="top" width="45%">Equivalent to [[:word:]]. </td>
<td width="5%"> </td>
</tr>
<tr>
<td width="5%"> </td>
<td valign="top" width="45%">\W </td>
<td valign="top" width="45%">Equivalent to [^[:word:]]. </td>
<td width="5%"> </td>
</tr>
<tr>
<td width="5%"> </td>
<td valign="top" width="45%">\s </td>
<td valign="top" width="45%">Equivalent to [[:space:]]. </td>
<td width="5%"> </td>
</tr>
<tr>
<td width="5%"> </td>
<td valign="top" width="45%">\S </td>
<td valign="top" width="45%">Equivalent to [^[:space:]]. </td>
<td width="5%"> </td>
</tr>
<tr>
<td width="5%"> </td>
<td valign="top" width="45%">\d </td>
<td valign="top" width="45%">Equivalent to [[:digit:]]. </td>
<td width="5%"> </td>
</tr>
<tr>
<td width="5%"> </td>
<td valign="top" width="45%">\D </td>
<td valign="top" width="45%">Equivalent to [^[:digit:]]. </td>
<td width="5%"> </td>
</tr>
<tr>
<td width="5%"> </td>
<td valign="top" width="45%">\l </td>
<td valign="top" width="45%">Equivalent to [[:lower:]]. </td>
<td width="5%"> </td>
</tr>
<tr>
<td width="5%"> </td>
<td valign="top" width="45%">\L </td>
<td valign="top" width="45%">Equivalent to [^[:lower:]]. </td>
<td width="5%"> </td>
</tr>
<tr>
<td width="5%"> </td>
<td valign="top" width="45%">\u </td>
<td valign="top" width="45%">Equivalent to [[:upper:]]. </td>
<td width="5%"> </td>
</tr>
<tr>
<td width="5%"> </td>
<td valign="top" width="45%">\U </td>
<td valign="top" width="45%">Equivalent to [^[:upper:]]. </td>
<td width="5%"> </td>
</tr>
<tr>
<td width="5%"> </td>
<td valign="top" width="45%">\C </td>
<td valign="top" width="45%">Any single character,
equivalent to '.'. </td>
<td width="5%"> </td>
</tr>
<tr>
<td width="5%"> </td>
<td valign="top" width="45%">\X </td>
<td valign="top" width="45%">Match any Unicode combining
character sequence, for example "a\x 0301" (a
letter a with an acute). </td>
<td width="5%"> </td>
</tr>
<tr>
<td width="5%"> </td>
<td valign="top" width="45%">\Q </td>
<td valign="top" width="45%">The begin quote operator,
everything that follows is treated as a literal character
until a \E end quote operator is found. </td>
<td width="5%"> </td>
</tr>
<tr>
<td width="5%"> </td>
<td valign="top" width="45%">\E </td>
<td valign="top" width="45%">The end quote operator,
terminates a sequence begun with \Q. </td>
<td width="5%"> </td>
</tr>
</table>
<p><br>
</p>
<p><i>What gets matched?</i> </p>
<p>The regular expression library will match the first possible
matching string, if more than one string starting at a given
location can match then it matches the longest possible string,
unless the flag match_any is set, in which case the first match
encountered is returned. Use of the match_any option can reduce
the time taken to find the match - but is only useful if the user
is less concerned about what matched - for example it would not
be suitable for search and replace operations. In cases where
their are multiple possible matches all starting at the same
location, and all of the same length, then the match chosen is
the one with the longest first sub-expression, if that is the
same for two or more matches, then the second sub-expression will
be examined and so on. <br>
</p>
<hr>
<p><i>Copyright </i><a href="mailto:John_Maddock@compuserve.com"><i>Dr
John Maddock</i></a><i> 1998-2000 all rights reserved.</i> </p>
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