Using Patterns for Search and Replace

One of the niftiest uses of regular expressions involves not just searching for a pattern or extracting matches into lists or variables or whatever, but also replacing that pattern with some other string. This is equivalent to a search and replace in your favorite word processor or editor—only with all the power and flexibility that regular expressions give you.

To search for a pattern and then replace it with some other pattern, use this syntax:

s/pattern/replacement/

For this syntax the pattern is some form of regular expression; replacement is the string that will replace the match. A missing replacement will delete the match from the string. For example:

s/s+/ /   # replace one or more whitespace characters with a single space

As with regular patterns, this syntax will search and replace the string in $_ by default. Use the =~ operator to search a different location.

The search-and-replace syntax, as shown, will replace only the first match, and return 1. With the /g option (g stands for global) on the end, Perl will replace all instances of the pattern in the string with the replacement:

s/--/—/g # replace two dashes -- with an em dash code —

Also, as with regular patterns, you can use the /i option at the end to perform a search that's not case sensitive (although the replacement will not match case, so be careful):

s/a/b/gi;  # replace [Aa] with b, globally

Feel free to use parentheses and match variables inside search-and-replace patterns; they work just fine here:

s/^(S+)/=$1=/g  # put = signs around first word
s//^(S+)(s.*)(S+)$/$3†$2†$1/ # swap first and last words