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     Regular Expressions

     The patterns used in pattern matching  are  regular  expres-
     sions  such  as  those supplied in the Version 8 regexp rou-
     tines.  (In  fact,  the  routines  are  derived  from  Henry
     Spencer's  freely redistributable reimplementation of the V8
     routines.)  In addition, \w matches an alphanumeric  charac-
     ter  (including  "_")  and \W a nonalphanumeric.  Word boun-
     daries may be matched by \b, and non-boundaries  by  \B.   A
     whitespace character is matched by \s, non-whitespace by \S.
     A numeric character is matched by  \d,  non-numeric  by  \D.
     You  may  use \w, \s and \d within character classes.  Also,
     \n, \r, \f, \t and \NNN have their  normal  interpretations.
     Within character classes \b represents backspace rather than
     a word boundary.  Alternatives may be separated by  |.   The
     bracketing construct ( ... ) may also be used, in which case
     \<digit> matches the digit'th substring.   (Outside  of  the
     pattern,  always  use  $ instead of \ in front of the digit.
     The scope of $<digit> (and $`, $& and $') extends to the end
     of  the  enclosing BLOCK or eval string, or to the next pat-
     tern match with subexpressions.  The \<digit> notation some-
     times  works  outside the current pattern, but should not be
     relied upon.)  You may have as many parentheses as you wish.
     If  you have more than 9 substrings, the variables $10, $11,
     ... refer to the corresponding substring.  Within  the  pat-
     tern,  \10, \11, etc. refer back to substrings if there have
     been at least that many left parens  before  the  backrefer-
     ence.  Otherwise (for backward compatibilty) \10 is the same
     as \010, a backspace, and \11 the same as \011, a tab.   And
     so on.  (\1 through \9 are always backreferences.)

     $+ returns whatever the  last  bracket  match  matched.   $&
     returns  the  entire matched string.  ($0 used to return the
     same thing, but not any more.)  $` returns everything before
     the matched string.  $' returns everything after the matched
     string.  Examples:

          s/^([^ ]*) *([^ ]*)/$2 $1/;   # swap first two words

          if (/Time: (..):(..):(..)/) {
               $hours = $1;
               $minutes = $2;
               $seconds = $3;
          }

     By default, the ^ character is only guaranteed to  match  at
     the beginning of the string, the $ character only at the end
     (or before the newline at the end)  and  perl  does  certain
     optimizations  with  the assumption that the string contains
     only one line.  The behavior of ^ and $ on embedded newlines
     will  be  inconsistent.   You  may, however, wish to treat a
     string as a multi-line buffer, such that the  ^  will  match
     after any newline within the string, and $ will match before
     any newline.  At the cost of a little more overhead, you can
     do this by setting the variable $* to 1.  Setting it back to
     0 makes perl revert to its old behavior.

     To facilitate  multi-line  substitutions,  the  .  character
     never matches a newline (even when $* is 0).  In particular,
     the following leaves a newline on the $_ string:

          $_ = <STDIN>;
          s/.*(some_string).*/$1/;

     If the newline is unwanted, try one of

          s/.*(some_string).*\n/$1/;
          s/.*(some_string)[^\000]*/$1/;
          s/.*(some_string)(.|\n)*/$1/;
          chop; s/.*(some_string).*/$1/;
          /(some_string)/ && ($_ = $1);

     Any item of a regular expression may be followed with digits
     in  curly  brackets  of  the  form  {n,m}, where n gives the
     minimum number of times to match the item and  m  gives  the
     maximum.   The  form  {n} is equivalent to {n,n} and matches
     exactly n times.  The form {n,} matches  n  or  more  times.
     (If  a  curly  bracket  occurs  in  any other context, it is
     treated  as  a  regular  character.)   The  *  modifier   is
     equivalent  to {0,}, the + modifier to {1,} and the ? modif-
     ier to {0,1}.  There is no limit to the size of n or m,  but
     large numbers will chew up more memory.

     You will note that all backslashed  metacharacters  in  perl
     are  alphanumeric,  such  as  \b, \w, \n.  Unlike some other
     regular expression languages, there are no backslashed  sym-
     bols  that aren't alphanumeric.  So anything that looks like
     \\, \(, \), \<, \>, \{, or \} is  always  interpreted  as  a
     literal  character, not a metacharacter.  This makes it sim-
     ple to quote a string that you want to use for a pattern but
     that  you  are  afraid might contain metacharacters.  Simply
     quote all the non-alphanumeric characters:

          $pattern =~ s/(\W)/\\$1/g;