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><DIV
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><H1
><A
NAME="REGEXP"
></A
>Chapter 18. Regular Expressions</H1
><TABLE
BORDER="0"
WIDTH="100%"
CELLSPACING="0"
CELLPADDING="0"
CLASS="EPIGRAPH"
><TR
><TD
WIDTH="45%"
>&nbsp;</TD
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WIDTH="45%"
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VALIGN="TOP"
><I
><P
><I
>. . . the intellectual activity associated with software
        development is largely one of gaining insight.</I
></P
><P
><I
>--Stowe Boyd</I
></P
></I
></TD
></TR
></TABLE
><P
><A
NAME="REGEXREF"
></A
></P
><P
>To fully utilize the power of shell scripting, you need to
	master Regular Expressions. Certain commands
	and utilities commonly used in scripts, such
	as <A
HREF="textproc.html#GREPREF"
>grep</A
>, <A
HREF="moreadv.html#EXPRREF"
>expr</A
>, <A
HREF="sedawk.html#SEDREF"
>sed</A
>
	and <A
HREF="awk.html#AWKREF"
>awk</A
>, interpret and use REs. As of
	<A
HREF="bashver3.html#BASH3REF"
>version 3</A
>, Bash has acquired its
	own <A
HREF="bashver3.html#REGEXMATCHREF"
>RE-match operator</A
>:
	<B
CLASS="COMMAND"
>=~</B
>.</P
><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="AEN17129"
></A
>18.1. A Brief Introduction to Regular Expressions</H1
><P
>An expression is a string of characters. Those characters
	  having an interpretation above and beyond their literal
	  meaning are called <I
CLASS="FIRSTTERM"
>metacharacters</I
>.
	  A quote symbol, for example, may denote speech by a person,
	  <I
CLASS="FIRSTTERM"
>ditto</I
>, or a meta-meaning

	    <A
NAME="AEN17134"
HREF="#FTN.AEN17134"
>[1]</A
>

	  for the symbols that follow. Regular Expressions are sets
	  of characters and/or metacharacters that match (or specify)
	  patterns.</P
><P
>A Regular Expression contains one or more of the
	following:</P
><UL
><LI
><P
><I
CLASS="FIRSTTERM"
>A character set</I
>. These are the
	      characters retaining their literal meaning. The
	      simplest type of Regular Expression consists
	      <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>only</I
></SPAN
> of a character set, with no
	      metacharacters.</P
></LI
><LI
><P
><A
NAME="ANCHORREF"
></A
></P
><P
><I
CLASS="FIRSTTERM"
>An anchor</I
>. These designate
	      (<I
CLASS="FIRSTTERM"
>anchor</I
>) the position in the line of
	      text that the RE is to match. For example, <SPAN
CLASS="TOKEN"
>^</SPAN
>,
	      and <SPAN
CLASS="TOKEN"
>$</SPAN
> are anchors.</P
></LI
><LI
><P
><I
CLASS="FIRSTTERM"
>Modifiers</I
>. These expand or narrow
	      (<I
CLASS="FIRSTTERM"
>modify</I
>) the range of text the RE is
	      to match. Modifiers include the asterisk, brackets, and
	      the backslash.</P
></LI
></UL
><P
>The main uses for Regular Expressions
	  (<I
CLASS="FIRSTTERM"
>RE</I
>s) are text searches and string
	  manipulation. An RE <I
CLASS="FIRSTTERM"
>matches</I
> a single
	  character or a set of characters -- a string or a part of
	  a string.</P
><UL
><LI
><P
><A
NAME="ASTERISKREG"
></A
>The asterisk --
	    <SPAN
CLASS="TOKEN"
>*</SPAN
> -- matches any number of
	      repeats of the character string or RE preceding it,
	      including <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>zero</I
></SPAN
> instances.</P
><P
><SPAN
CLASS="QUOTE"
>"1133*"</SPAN
> matches <TT
CLASS="REPLACEABLE"
><I
>11 +
	      one or more 3's</I
></TT
>:
	      <TT
CLASS="REPLACEABLE"
><I
>113</I
></TT
>, <TT
CLASS="REPLACEABLE"
><I
>1133</I
></TT
>,
	      <TT
CLASS="REPLACEABLE"
><I
>1133333</I
></TT
>, and so forth.</P
></LI
><LI
><P
><A
NAME="REGEXDOT"
></A
>The <I
CLASS="FIRSTTERM"
>dot</I
>
	    -- <SPAN
CLASS="TOKEN"
>.</SPAN
> -- matches
	      any one character, except a newline.
	        <A
NAME="AEN17189"
HREF="#FTN.AEN17189"
>[2]</A
>  
	    </P
><P
><SPAN
CLASS="QUOTE"
>"13."</SPAN
> matches <TT
CLASS="REPLACEABLE"
><I
>13 + at
	     least one of any character (including a
	     space)</I
></TT
>: <TT
CLASS="REPLACEABLE"
><I
>1133</I
></TT
>,
	     <TT
CLASS="REPLACEABLE"
><I
>11333</I
></TT
>, but not
	     <TT
CLASS="REPLACEABLE"
><I
>13</I
></TT
> (additional character
	     missing).</P
><P
>See <A
HREF="textproc.html#CWSOLVER"
>Example 16-18</A
> for a demonstration
	       of <I
CLASS="FIRSTTERM"
>dot single-character</I
>
	       matching.</P
></LI
><LI
><P
><A
NAME="CARETREF"
></A
>The caret -- <SPAN
CLASS="TOKEN"
>^</SPAN
>
	      -- matches the beginning of a line, but sometimes, depending
	      on context, negates the meaning of a set of characters in
	      an RE.</P
></LI
><LI
><P
><A
NAME="DOLLARSIGNREF"
></A
></P
><P
>The dollar sign -- <SPAN
CLASS="TOKEN"
>$</SPAN
> -- at the end of an
	      RE matches the end of a line.</P
><P
><SPAN
CLASS="QUOTE"
>"XXX$"</SPAN
> matches <SPAN
CLASS="TOKEN"
>XXX</SPAN
> at the
	      end of a line.</P
><P
><SPAN
CLASS="QUOTE"
>"^$"</SPAN
> matches blank lines.</P
></LI
><LI
><P
><A
NAME="BRACKETSREF"
></A
></P
><P
>Brackets -- <SPAN
CLASS="TOKEN"
>[...]</SPAN
> -- enclose a set of characters
	      to match in a single RE.</P
><P
><SPAN
CLASS="QUOTE"
>"[xyz]"</SPAN
> matches any one of the characters
	      <TT
CLASS="REPLACEABLE"
><I
>x</I
></TT
>, <TT
CLASS="REPLACEABLE"
><I
>y</I
></TT
>,
	      or <TT
CLASS="REPLACEABLE"
><I
>z</I
></TT
>.</P
><P
><SPAN
CLASS="QUOTE"
>"[c-n]"</SPAN
> matches any one of the
	      characters in the range <TT
CLASS="REPLACEABLE"
><I
>c</I
></TT
>
	      to <TT
CLASS="REPLACEABLE"
><I
>n</I
></TT
>.</P
><P
><SPAN
CLASS="QUOTE"
>"[B-Pk-y]"</SPAN
> matches any one of the
	      characters in the ranges <TT
CLASS="REPLACEABLE"
><I
>B</I
></TT
>
	      to <TT
CLASS="REPLACEABLE"
><I
>P</I
></TT
>
	      and <TT
CLASS="REPLACEABLE"
><I
>k</I
></TT
> to
	      <TT
CLASS="REPLACEABLE"
><I
>y</I
></TT
>.</P
><P
><SPAN
CLASS="QUOTE"
>"[a-z0-9]"</SPAN
> matches any single lowercase
	      letter or any digit.</P
><P
><SPAN
CLASS="QUOTE"
>"[^b-d]"</SPAN
> matches any character
	      <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>except</I
></SPAN
> those in
	      the range <TT
CLASS="REPLACEABLE"
><I
>b</I
></TT
> to
	      <TT
CLASS="REPLACEABLE"
><I
>d</I
></TT
>. This is an instance of
	      <SPAN
CLASS="TOKEN"
>^</SPAN
> negating or inverting the meaning
	      of the following RE (taking on a role similar to
	      <SPAN
CLASS="TOKEN"
>!</SPAN
> in a different context).</P
><P
>Combined sequences of bracketed characters match  
	      common word patterns. <SPAN
CLASS="QUOTE"
>"[Yy][Ee][Ss]"</SPAN
> matches
	      <TT
CLASS="REPLACEABLE"
><I
>yes</I
></TT
>, <TT
CLASS="REPLACEABLE"
><I
>Yes</I
></TT
>,
	      <TT
CLASS="REPLACEABLE"
><I
>YES</I
></TT
>, <TT
CLASS="REPLACEABLE"
><I
>yEs</I
></TT
>,
	      and so forth.
	      <SPAN
CLASS="QUOTE"
>"[0-9][0-9][0-9]-[0-9][0-9]-[0-9][0-9][0-9][0-9]"</SPAN
>
	      matches any Social Security number.</P
></LI
><LI
><P
><A
NAME="REGEXBS"
></A
></P
><P
>The backslash -- <SPAN
CLASS="TOKEN"
>\</SPAN
> -- <A
HREF="escapingsection.html#ESCP"
>escapes</A
> a special character, which
              means that character gets interpreted literally (and is
              therefore no longer <I
CLASS="FIRSTTERM"
>special</I
>).</P
><P
>A <SPAN
CLASS="QUOTE"
>"\$"</SPAN
> reverts back to its
	       literal meaning of <SPAN
CLASS="QUOTE"
>"$"</SPAN
>, rather than its
	       RE meaning of end-of-line. Likewise a <SPAN
CLASS="QUOTE"
>"\\"</SPAN
>
	       has the literal meaning of <SPAN
CLASS="QUOTE"
>"\"</SPAN
>.</P
></LI
><LI
><P
><A
NAME="ANGLEBRAC"
></A
></P
><P
><A
HREF="escapingsection.html#ESCP"
>Escaped</A
> <SPAN
CLASS="QUOTE"
>"angle
	      brackets"</SPAN
> -- <SPAN
CLASS="TOKEN"
>\&#60;...\&#62;</SPAN
> -- mark word
	      boundaries.</P
><P
>The angle brackets must be escaped, since otherwise
	      they have only their literal character meaning.</P
><P
><SPAN
CLASS="QUOTE"
>"\&#60;the\&#62;"</SPAN
> matches the word
	      <SPAN
CLASS="QUOTE"
>"the,"</SPAN
> but not the words <SPAN
CLASS="QUOTE"
>"them,"</SPAN
>
	      <SPAN
CLASS="QUOTE"
>"there,"</SPAN
> <SPAN
CLASS="QUOTE"
>"other,"</SPAN
> etc.</P
><P
>	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>cat textfile</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>This is line 1, of which there is only one instance.
 This is the only instance of line 2.
 This is line 3, another line.
 This is line 4.</TT
>
 
 
 <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>grep 'the' textfile</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>This is line 1, of which there is only one instance.
 This is the only instance of line 2.
 This is line 3, another line.</TT
>
 
 
 <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>grep '\&#60;the\&#62;' textfile</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>This is the only instance of line 2.</TT
>
 	      </PRE
></TD
></TR
></TABLE
>
	    </P
></LI
></UL
><TABLE
CLASS="SIDEBAR"
BORDER="1"
CELLPADDING="5"
><TR
><TD
><DIV
CLASS="SIDEBAR"
><A
NAME="AEN17316"
></A
><P
>The only way to be certain that a particular RE works is to
	    test it.</P
><P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;TEST FILE: tstfile                          # No match.
   2&nbsp;                                            # No match.
   3&nbsp;Run   grep "1133*"  on this file.           # Match.
   4&nbsp;                                            # No match.
   5&nbsp;                                            # No match.
   6&nbsp;This line contains the number 113.          # Match.
   7&nbsp;This line contains the number 13.           # No match.
   8&nbsp;This line contains the number 133.          # No match.
   9&nbsp;This line contains the number 1133.         # Match.
  10&nbsp;This line contains the number 113312.       # Match.
  11&nbsp;This line contains the number 1112.         # No match.
  12&nbsp;This line contains the number 113312312.    # Match.
  13&nbsp;This line contains no numbers at all.       # No match.</PRE
></TD
></TR
></TABLE
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>grep "1133*" tstfile</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>Run   grep "1133*"  on this file.           # Match.
 This line contains the number 113.          # Match.
 This line contains the number 1133.         # Match.
 This line contains the number 113312.       # Match.
 This line contains the number 113312312.    # Match.</TT
>
 	      </PRE
></TD
></TR
></TABLE
></DIV
></TD
></TR
></TABLE
><UL
><LI
STYLE="list-style-type: square"
><DIV
CLASS="FORMALPARA"
><P
><B
><A
NAME="EXTREGEX"
></A
>Extended REs. </B
>Additional metacharacters added to the basic set. Used
		in <A
HREF="textproc.html#EGREPREF"
>egrep</A
>,
		<A
HREF="awk.html#AWKREF"
>awk</A
>, and <A
HREF="wrapper.html#PERLREF"
>Perl</A
>.</P
></DIV
></LI
><LI
><P
><A
NAME="QUEXREGEX"
></A
></P
><P
>The question mark -- <SPAN
CLASS="TOKEN"
>?</SPAN
> -- matches zero or
	      one of the previous RE. It is generally used for matching
	      single characters.</P
></LI
><LI
><P
><A
NAME="PLUSREF"
></A
></P
><P
>The plus -- <SPAN
CLASS="TOKEN"
>+</SPAN
> -- matches one or more of the
	    previous RE. It serves a role similar to the <SPAN
CLASS="TOKEN"
>*</SPAN
>, but
	    does <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>not</I
></SPAN
> match zero occurrences.</P
><P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;# GNU versions of sed and awk can use "+",
   2&nbsp;# but it needs to be escaped.
   3&nbsp;
   4&nbsp;echo a111b | sed -ne '/a1\+b/p'
   5&nbsp;echo a111b | grep 'a1\+b'
   6&nbsp;echo a111b | gawk '/a1+b/'
   7&nbsp;# All of above are equivalent.
   8&nbsp;
   9&nbsp;# Thanks, S.C.</PRE
></TD
></TR
></TABLE
></P
><P
><A
NAME="ESCPCB"
></A
></P
></LI
><LI
><P
><A
HREF="escapingsection.html#ESCP"
>Escaped</A
> <SPAN
CLASS="QUOTE"
>"curly
	      brackets"</SPAN
> -- <SPAN
CLASS="TOKEN"
>\{ \}</SPAN
> -- indicate the number
	      of occurrences of a preceding RE to match.</P
><P
>It is necessary to escape the curly brackets since
	      they have only their literal character meaning
	      otherwise. This usage is technically not part of the basic
	      RE set.</P
><P
><SPAN
CLASS="QUOTE"
>"[0-9]\{5\}"</SPAN
> matches exactly five digits
	      (characters in the range of 0 to 9).</P
><DIV
CLASS="NOTE"
><TABLE
CLASS="NOTE"
WIDTH="90%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/note.png"
HSPACE="5"
ALT="Note"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>Curly brackets are not available as an RE in the
	      <SPAN
CLASS="QUOTE"
>"classic"</SPAN
> (non-POSIX compliant) version
	      of <A
HREF="awk.html#AWKREF"
>awk</A
>.
	      <A
NAME="GNUGAWK"
></A
>However, the GNU extended version
	      of <I
CLASS="FIRSTTERM"
>awk</I
>, <B
CLASS="COMMAND"
>gawk</B
>,
	      has the <TT
CLASS="OPTION"
>--re-interval</TT
> option that permits
	      them (without being escaped).</P
><P
>	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>echo 2222 | gawk --re-interval '/2{3}/'</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>2222</TT
>
 	      </PRE
></TD
></TR
></TABLE
>
	    </P
><P
><B
CLASS="COMMAND"
>Perl</B
> and some
	      <B
CLASS="COMMAND"
>egrep</B
> versions do not require escaping
	      the curly brackets.</P
></TD
></TR
></TABLE
></DIV
></LI
><LI
><P
><A
NAME="PARENGRPS"
></A
></P
><P
>Parentheses -- <B
CLASS="COMMAND"
>( )</B
> -- enclose a group of
	      REs. They are useful with the following
	      <SPAN
CLASS="QUOTE"
>"<SPAN
CLASS="TOKEN"
>|</SPAN
>"</SPAN
> operator and in <A
HREF="manipulatingvars.html#EXPRPAREN"
>substring extraction</A
> using <A
HREF="moreadv.html#EXPRREF"
>expr</A
>.</P
></LI
><LI
><P
>The -- <B
CLASS="COMMAND"
>|</B
> -- <SPAN
CLASS="QUOTE"
>"or"</SPAN
> RE operator
	      matches any of a set of alternate characters.</P
><P
>	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>egrep 're(a|e)d' misc.txt</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>People who read seem to be better informed than those who do not.
 The clarinet produces sound by the vibration of its reed.</TT
>
 	      </PRE
></TD
></TR
></TABLE
>
	      </P
></LI
></UL
><DIV
CLASS="NOTE"
><TABLE
CLASS="NOTE"
WIDTH="100%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/note.png"
HSPACE="5"
ALT="Note"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>Some versions of <B
CLASS="COMMAND"
>sed</B
>,
	      <B
CLASS="COMMAND"
>ed</B
>, and <B
CLASS="COMMAND"
>ex</B
> support
	      escaped versions of the extended Regular Expressions
	      described above, as do the GNU utilities.</P
></TD
></TR
></TABLE
></DIV
><UL
><LI
STYLE="list-style-type: square"
><DIV
CLASS="FORMALPARA"
><P
><B
><A
NAME="POSIXREF"
></A
>POSIX Character Classes. </B
><TT
CLASS="USERINPUT"
><B
>[:class:]</B
></TT
></P
></DIV
><P
>This is an alternate method of specifying a range of
	      characters to match.</P
></LI
><LI
><P
><TT
CLASS="USERINPUT"
><B
>[:alnum:]</B
></TT
> matches alphabetic or
	      numeric characters. This is equivalent to
	      <TT
CLASS="USERINPUT"
><B
>A-Za-z0-9</B
></TT
>.</P
></LI
><LI
><P
><TT
CLASS="USERINPUT"
><B
>[:alpha:]</B
></TT
> matches alphabetic
	      characters. This is equivalent to
	      <TT
CLASS="USERINPUT"
><B
>A-Za-z</B
></TT
>.</P
></LI
><LI
><P
><TT
CLASS="USERINPUT"
><B
>[:blank:]</B
></TT
> matches a space or a
	      tab.</P
></LI
><LI
><P
><TT
CLASS="USERINPUT"
><B
>[:cntrl:]</B
></TT
> matches control
	      characters.</P
></LI
><LI
><P
><TT
CLASS="USERINPUT"
><B
>[:digit:]</B
></TT
> matches (decimal)
	      digits. This is equivalent to
	      <TT
CLASS="USERINPUT"
><B
>0-9</B
></TT
>.</P
></LI
><LI
><P
><TT
CLASS="USERINPUT"
><B
>[:graph:]</B
></TT
> (graphic printable
		    characters). Matches characters in the range of <A
HREF="special-chars.html#ASCIIDEF"
>ASCII</A
> 33 - 126. This is
		    the same as <TT
CLASS="USERINPUT"
><B
>[:print:]</B
></TT
>, below,
		    but excluding the space character.</P
></LI
><LI
><P
><TT
CLASS="USERINPUT"
><B
>[:lower:]</B
></TT
> matches lowercase
	      alphabetic characters. This is equivalent to
	      <TT
CLASS="USERINPUT"
><B
>a-z</B
></TT
>.</P
></LI
><LI
><P
><TT
CLASS="USERINPUT"
><B
>[:print:]</B
></TT
> (printable
	      characters). Matches characters in the range of ASCII 32 -
	      126. This is the same as <TT
CLASS="USERINPUT"
><B
>[:graph:]</B
></TT
>,
	      above, but adding the space character.</P
></LI
><LI
><P
><A
NAME="WSPOSIX"
></A
><TT
CLASS="USERINPUT"
><B
>[:space:]</B
></TT
>
	      matches whitespace characters (space and horizontal
	      tab).</P
></LI
><LI
><P
><TT
CLASS="USERINPUT"
><B
>[:upper:]</B
></TT
> matches uppercase
	      alphabetic characters. This is equivalent to
	      <TT
CLASS="USERINPUT"
><B
>A-Z</B
></TT
>.</P
></LI
><LI
><P
><TT
CLASS="USERINPUT"
><B
>[:xdigit:]</B
></TT
> matches hexadecimal
	      digits. This is equivalent to
	      <TT
CLASS="USERINPUT"
><B
>0-9A-Fa-f</B
></TT
>.</P
><DIV
CLASS="IMPORTANT"
><TABLE
CLASS="IMPORTANT"
WIDTH="90%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/important.png"
HSPACE="5"
ALT="Important"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>POSIX character classes generally require quoting
	      or <A
HREF="tests.html#DBLBRACKETS"
>double brackets</A
>
	      ([[ ]]).</P
></TD
></TR
></TABLE
></DIV
><P
>	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>grep [[:digit:]] test.file</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>abc=723</TT
>
 	      </PRE
></TD
></TR
></TABLE
>
	    </P
><P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;# ...
   2&nbsp;if [[ $arow =~ [[:digit:]] ]]   #  Numerical input?
   3&nbsp;then       #  POSIX char class
   4&nbsp;  if [[ $acol =~ [[:alpha:]] ]] # Number followed by a letter? Illegal!
   5&nbsp;# ...
   6&nbsp;# From ktour.sh example script.</PRE
></TD
></TR
></TABLE
>
	    </P
><P
>These character classes may even be used with <A
HREF="globbingref.html"
>globbing</A
>, to a limited
	      extent.</P
><P
>	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>ls -l ?[[:digit:]][[:digit:]]?</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>-rw-rw-r--    1 bozo  bozo         0 Aug 21 14:47 a33b</TT
>
 	      </PRE
></TD
></TR
></TABLE
>
	    </P
><P
>POSIX character classes are used in
	      <A
HREF="textproc.html#EX49"
>Example 16-21</A
> and <A
HREF="textproc.html#LOWERCASE"
>Example 16-22</A
>.</P
></LI
></UL
><P
><A
HREF="sedawk.html#SEDREF"
>Sed</A
>, <A
HREF="awk.html#AWKREF"
>awk</A
>, and <A
HREF="wrapper.html#PERLREF"
>Perl</A
>, used as filters in scripts, take
	  REs as arguments when "sifting" or transforming files or I/O
	  streams. See <A
HREF="contributed-scripts.html#BEHEAD"
>Example A-12</A
> and <A
HREF="contributed-scripts.html#TREE"
>Example A-16</A
>
	  for illustrations of this.</P
><P
>The standard reference on this complex topic is Friedl's
	  <I
CLASS="CITETITLE"
>Mastering Regular
	  Expressions</I
>. <I
CLASS="CITETITLE"
>Sed &#38;
	  Awk</I
>, by Dougherty and Robbins, also gives a very
	  lucid treatment of REs. See the <A
HREF="biblio.html"
><I
>Bibliography</I
></A
> for
	  more information on these books.</P
></DIV
></DIV
><H3
CLASS="FOOTNOTES"
>Notes</H3
><TABLE
BORDER="0"
CLASS="FOOTNOTES"
WIDTH="100%"
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN17134"
HREF="regexp.html#AEN17134"
>[1]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
><A
NAME="METAMEANINGREF"
></A
>A
	    <I
CLASS="FIRSTTERM"
>meta-meaning</I
> is the meaning of a
	    term or expression on a higher level of abstraction. For
	    example, the <I
CLASS="FIRSTTERM"
>literal</I
> meaning
	    of <I
CLASS="FIRSTTERM"
>regular expression</I
> is an
	    ordinary expression that conforms to accepted usage. The
	    <I
CLASS="FIRSTTERM"
>meta-meaning</I
> is drastically different,
	    as discussed at length in this chapter.</P
></TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN17189"
HREF="regexp.html#AEN17189"
>[2]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
>Since <A
HREF="sedawk.html#SEDREF"
>sed</A
>, <A
HREF="awk.html#AWKREF"
>awk</A
>, and <A
HREF="textproc.html#GREPREF"
>grep</A
> process single lines, there
		  will usually not be a newline to match. In those cases where
		  there is a newline in a multiple line expression, the dot
		  will match the newline.
	            <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;
   3&nbsp;sed -e 'N;s/.*/[&#38;]/' &#60;&#60; EOF   # Here Document
   4&nbsp;line1
   5&nbsp;line2
   6&nbsp;EOF
   7&nbsp;# OUTPUT:
   8&nbsp;# [line1
   9&nbsp;# line2]
  10&nbsp;
  11&nbsp;
  12&nbsp;
  13&nbsp;echo
  14&nbsp;
  15&nbsp;awk '{ $0=$1 "\n" $2; if (/line.1/) {print}}' &#60;&#60; EOF
  16&nbsp;line 1
  17&nbsp;line 2
  18&nbsp;EOF
  19&nbsp;# OUTPUT:
  20&nbsp;# line
  21&nbsp;# 1
  22&nbsp;
  23&nbsp;
  24&nbsp;# Thanks, S.C.
  25&nbsp;
  26&nbsp;exit 0</PRE
></TD
></TR
></TABLE
></P
></TD
></TR
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