Learning Perl, 7th Edition

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 ...