m/PATTERN/gio

     /PATTERN/gio

             Searches a string for a pattern match,  and  returns
             true  (1)  or false ('').  If no string is specified
             via  the  =~  or  !~  operator,  the  $_  string  is
             searched.  (The string specified with =~ need not be
             an lvalue--it may be the  result  of  an  expression
             evaluation,   but   remember  the  =~  binds  rather
             tightly.)  See also the section on  regular  expres-
             sions.

             If / is  the  delimiter  then  the  initial  'm'  is
             optional.   With  the  'm'  you  can use any pair of
             non-alphanumeric characters as delimiters.  This  is
             particularly  useful  for  matching  Unix path names
             that contain '/'.  If the final  delimiter  is  fol-
             lowed  by  the  optional letter 'i', the matching is
             done in a case-insensitive manner.  PATTERN may con-
             tain  references  to scalar variables, which will be
             interpolated (and the pattern recompiled) every time
             the  pattern search is evaluated.  (Note that $) and
             $| may not be interpolated because  they  look  like
             end-of-string tests.)  If you want such a pattern to
             be compiled only once, add an "o" after the trailing
             delimiter.   This avoids expensive run-time recompi-
             lations, and is useful when the value you are inter-
             polating  won't  change over the life of the script.

             If the PATTERN evaluates to a null string, the  most
             recent   successful   regular   expression  is  used
             instead.

             If used in a context that requires an array value, a
             pattern  match  returns  an  array consisting of the
             subexpressions matched by  the  parentheses  in  the
             pattern, i.e. ($1, $2, $3...).  It does NOT actually
             set $1, $2, etc. in this case, nor does it  set  $+,
             $`,  $&  or $'.  If the match fails, a null array is
             returned.  If the match succeeds, but there were  no
             parentheses, an array value of (1) is returned.

             Examples:

                 open(tty, '/dev/tty');
                 <tty> =~ /^y/i && do foo();    # do foo if desired

                 if (/Version: *([0-9.]*)/) { $version = $1; }

                 next if m#^/usr/spool/uucp#;

                 # poor man's grep
                 $arg = shift;
                 while (<>) {
                      print if /$arg/o;    # compile only once
                 }

                 if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/))

             This last example splits $foo  into  the  first  two
             words  and  the  remainder  of the line, and assigns
             those three fields to $F1, $F2 and $Etc.  The condi-
             tional  is true if any variables were assigned, i.e.
             if the pattern matched.

             The   "g"   modifier   specifies   global    pattern
             matching--that  is, matching as many times as possi-
             ble within the string.  How it  behaves  depends  on
             the context.  In an array context, it returns a list
             of all the substrings matched by all the parentheses
             in   the   regular  expression.   If  there  are  no
             parentheses, it returns a list of  all  the  matched
             strings,  as  if  there  were parentheses around the
             whole pattern.  In a  scalar  context,  it  iterates
             through  the  string,  returning  TRUE  each time it
             matches, and FALSE when it eventually  runs  out  of
             matches.   (In  other  words,  it remembers where it
             left off last time and restarts the search  at  that
             point.)   It presumes that you have not modified the
             string since the last match.  Modifying  the  string
             between  matches  may  result in undefined behavior.

             (You can actually get away with  in-place  modifica-
             tions  via substr() that do not change the length of
             the entire string.  In general, however, you  should
             be using s///g for such modifications.)  Examples:

                  # array context
                  ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);

                  # scalar context
                  $/ = 1; $* = 1;
                  while ($paragraph = <>) {
                      while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) {
                       $sentences++;
                      }
                  }
                  print "$sentences\n";