Programming Perl, 4th Edition

The s/// Operator (Substitution)

s/PATTERN/REPLACEMENT/modifiers

 LVALUE =~ s/PATTERN/REPLACEMENT/modifiers
 RVALUE =~ s/PATTERN/REPLACEMENT/rmodifiers

This operator searches a string for PATTERN and, if found, replaces the matched substring with the REPLACEMENT text. If PATTERN is a null string, the last successfully executed regular expression is used instead.

$lotr = $hobbit;           # Just copy The Hobbit
$lotr =~ s/Bilbo/Frodo/g;  #   and write a sequel the easy way.

If the /r modifier is used, the return value of an s/// operation is the result string, and the target string is left unchanged. Without the /r modifier, the return value of an s/// operation (in scalar and list context alike) is the number of times it succeeded—which can be more than once if used with the /g modifier, as described earlier. On failure, since it substituted zero times, it returns false (""), which is numerically equivalent to 0.^[93]

if ($lotr =~ s/Bilbo/Frodo/) { say "Successfully wrote sequel." }
$change_count = $lotr =~ s/Bilbo/Frodo/g;

Normally, everything matched by the PATTERN is discarded on each substitution, but you can “keep” part of that by including \K in your pattern:

$tales_of_Rohan =~ s/Éo\Kmer/wyn/g; # rewriting history

The replacement portion is treated as a double-quoted string. You may use any of the dynamically scoped pattern variables described earlier ($`, $&, $', $1, $2, and so on) in the replacement string, as well as any other double-quote gizmos you care to employ. For instance, here’s an example ...