Computer Science & Perl Programming

Part II. Regular Expressions

Chapter 13

Chapter 14

Chapter 15

Chapter 16

Chapter 17

Perl lets you search huge collections of data with its built-in support for pattern-matching. To match a pattern, you create a regular expression, and test whether a string contains the pattern with the =~ and !~ operators, like so:

	print "Match!" if $greeting =~ /Hello/;

If $greeting contains the five-character sequence Hello, this code snippet prints Match!.

Regular expressions constitute a little language inside Perl, with their own peculiar syntax inherited from history. From a computer scientist’s point of view, Perl’s regular expressions aren’t even regular; features like \1 and \2, which allow you to refer to portions of already-matched text inside a pattern, aren’t permitted in “traditional” regular expressions.

This section won’t give you a tour of all of Perl’s regular expression metacharacters and assertions. For that, consult the perlre online documentation, or another Perl book. Instead, the five chapters here help you understand how Perl’s regular expressions work under the hood.

Four of the five articles are written by the world’s foremost regex guru: Jeffrey Friedl, author of O’Reilly’s Mastering Regular Expressions. His often-cited article Understanding Regular Expressions, Part I explains backtracking, one of the key concepts in regular expressions that cleanly separates the beginners from experts. In Understanding Regular Expressions, Part II, Jeffrey takes a simple problem—matching two ...