Chapter 9. Processing Text with Regular Expressions

You can use regular expressions to change text too. So far we’ve only shown you how to match a pattern, and now we’ll show you how to use patterns to locate the parts of strings that you want to change.

Substitutions with s///

If you think of the m// pattern match as being like your word processor’s “search” feature, the “search and replace” feature would be Perl’s s/// substitution operator. This simply replaces whatever part of a variable matches the pattern with a replacement string:

$_ = "He's out bowling with Barney tonight.";
s/Barney/Fred/;  # Replace Barney with Fred
print "$_\n";
Note

Unlike m//, which can match against any string expression, s/// is modifying data that must therefore be contained in what’s known as an lvalue. This is nearly always a variable, but could be anything you can use on the left side of an assignment operator.

If the match fails, nothing happens, and the variable is untouched:

# Continuing from previous; $_ has "He's out bowling with Fred tonight."
s/Wilma/Betty/;  # Replace Wilma with Betty (fails)

Of course, both the pattern and the replacement string could be more complex. Here, the replacement string uses the first capture variable, $1, which is set by the pattern match:

s/with (\w+)/against $1's team/;
print "$_\n";  # says "He's out bowling against Fred's team tonight."

Here are some other possible substitutions. These are here only as samples; in the real world, it would not be typical to do so many ...

Get Learning Perl, 7th Edition now with O’Reilly online learning.

O’Reilly members experience live online training, plus books, videos, and digital content from 200+ publishers.