Learning Perl, Fourth Edition

Chapter 7. In the World of Regular Expressions

Perl has many features that set it apart from other languages. Of all those features, one of the most important is its strong support for regular expressions. These allow fast, flexible, and reliable string handling.

But that power comes at a price. Regular expressions are tiny programs in their own special language, built inside Perl. (Yes, you’re about to learn another programming language.^[174] Fortunately, it’s a simple one.) In this chapter, you’ll visit the world of regular expressions, where (mostly) you can forget about the world of Perl. In the next chapter, we’ll show you where this world fits into Perl’s world.

Regular expressions aren’t merely part of Perl; they’re also found in sed and awk, procmail, grep, most programmers’ text editors like vi and emacs, and in more esoteric places. If you’ve seen some of these, you’re ahead of the game. Keep watching, and you’ll see many more tools that use or support regular expressions, such as search engines on the Web (often written in Perl), email clients, and others. The bad news is that everybody’s regular expressions have slightly different syntax, so you may need to learn to include or omit an occasional backslash.

What Are Regular Expressions?

A regular expression, often called a pattern in Perl, is a template that matches or doesn’t match a given string.^[175] An infinite number of possible text strings exist, and a given pattern divides that infinite set into two groups: the ones ...