s/PATTERN/REPLACEMENT/gieo

             Searches a string  for  a  pattern,  and  if  found,
             replaces  that pattern with the replacement text and
             returns the number of substitutions made.  Otherwise
             it  returns  false (0).  The "g" is optional, and if
             present,  indicates  that  all  occurrences  of  the
             pattern  are  to  be  replaced.   The  "i"  is  also
             optional, and if present, indicates that matching is
             to be done in a case-insensitive manner.  The "e" is
             likewise optional, and if  present,  indicates  that
             the  replacement  string  is  to  be evaluated as an
             expression  rather  than  just  as  a  double-quoted
             string.   Any non-alphanumeric delimiter may replace
             the slashes; if single quotes are used, no interpre-
             tation  is  done  on  the  replacement string (the e
             modifier overrides this, however); if backquotes are
             used, the replacement string is a command to execute
             whose output will be used as the actual  replacement
             text.   If  no  string is specified via the =~ or !~
             operator, the $_ string is  searched  and  modified.
             (The string specified with =~ must be a scalar vari-
             able, an array element, or an assignment to  one  of
             those, i.e. an lvalue.)  If the pattern contains a $
             that looks like a variable rather  than  an  end-of-
             string  test, the variable will be interpolated into
             the pattern at run-time.  If you only want the  pat-
             tern  compiled  once  the first time the variable is
             interpolated, add an "o" at the end.  If the PATTERN
             evaluates to a null string, the most recent success-
             ful regular expression is used  instead.   See  also
             the section on regular expressions.  Examples:

                 s/\bgreen\b/mauve/g;      # don't change wintergreen

                 $path =~ s|/usr/bin|/usr/local/bin|;

                 s/Login: $foo/Login: $bar/; # run-time pattern

                 ($foo = $bar) =~ s/bar/foo/;

                 $_ = 'abc123xyz';
                 s/\d+/$&*2/e;        # yields 'abc246xyz'
                 s/\d+/sprintf("%5d",$&)/e;     # yields 'abc  246xyz'
                 s/\w/$& x 2/eg;      # yields 'aabbcc  224466xxyyzz'

                 s/([^ ]*) *([^ ]*)/$2 $1/;     # reverse 1st two fields

             (Note the use of $ instead of \ in the last example.
             See section on regular expressions.)