4.8     Regular expressions

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So far we've been looking at string functions to help you manipulate strings, but what if you want more power - more control over your string manipulation?

Regular expressions, usually referred to as regexps, offer you that power, but are tricky to learn because they use complicated syntax to provide their power. The chances are that you have used regular expressions before, albeit perhaps on a fairly limited scale. For example, if you were at your shell prompt and typed "rm *.txt", you would expect all files ending with .txt to be removed.

Similarly, if you were to search your hard drive for all JPEG picture files, you would search for *.jpg - it would match foo.jpg, bar.jpg, fool.jpg, and barf.jpg. You've probably also used the ? regexp, which means "match precisely one character" - foo?.jpg would match only fool.jpg and not foobar.jpg.

Regular expressions offer a powerful and easy-to-use (if tricky to learn) way to perform powerful string matching and replacement functions. Regexps can:

• Replace text

• Test for a pattern within a string

• Extract a substring from within a string

We'll be looking at all three of these uses in this section, as well as providing a comprehensive list of the different expressions you can use to work with all kinds of strings.

Before we start, there are two things you should be aware of: firstly, only use regular expressions when you have to. The set of string functions we just looked at are much faster, much easier to read, and much less hassle to use. Secondly, PHP contains two ways to perform regular expressions, known as POSIX-extended and Perl Compatible Regular Expressions (PCRE). The PCRE functions are more powerful than the POSIX ones, and they are faster too, so we will be using the PCRE functions here.