7.1. 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. An infinite number of possible text strings exist, and a given pattern divides that infinite set into two groups: the ones that match and the ones that don't. There's never any kinda-sorta-almost-up-to-here wishy-washy matching: either it matches or it doesn't.
A pattern may match one possible string, two or three, a dozen, a hundred, or an infinite number. It may match all strings except for one, except for some, or except for an infinite number.[*] We've referred to regular expressions as being little programs in their own simple programming language. It's a simple language because the programs have one task: to look at a string and say "it matches" or "it doesn't match". That's all they do.
One of the places you're likely to have seen regular expressions is in the Unix grep command, which prints out text lines matching a given pattern. For example, if you wanted to see which lines in a given file mention flint and, somewhere later on the same line, stone, you might do something like this with the Unix grep command:
$ grep 'flint.*stone' chapter*.txt chapter3.txt:a piece of flint, a stone which may be used to start a fire by striking chapter3.txt:found obsidian, flint, granite, and small stones of basaltic rock, which chapter9.txt:a flintlock rifle in poor condition. The sandstone mantle held several
Don't confuse regular expressions with shell filename-matching patterns, called globs. A typical glob is what you use when you type *.pm to the Unix shell to match all filenames that end in .pm. The previous example uses a glob of chapter*.txt. (You may have noticed that you had to quote the pattern to prevent the shell from treating it like a glob.) Though globs use many of the same characters you use in regular expressions, those characters are used in different ways. You'll visit globs in Chapter 12.