Programming PHP, 3rd Edition

Regular Expressions

If you need more complex searching functionality than the previous methods provide, you can use regular expressions. A regular expression is a string that represents a pattern. The regular expression functions compare that pattern to another string and see if any of the string matches the pattern. Some functions tell you whether there was a match, while others make changes to the string.

There are three uses for regular expressions: matching, which can also be used to extract information from a string; substituting new text for matching text; and splitting a string into an array of smaller chunks. PHP has functions for all. For instance, preg_match() does a regular expression match.

Perl has long been considered the benchmark for powerful regular expressions. PHP uses a C library called pcre to provide almost complete support for Perl’s arsenal of regular expression features. Perl regular expressions act on arbitrary binary data, so you can safely match with patterns or strings that contain the NUL-byte (\x00).

The Basics

Most characters in a regular expression are literal characters, meaning that they match only themselves. For instance, if you search for the regular expression "/cow/" in the string "Dave was a cowhand", you get a match because "cow" occurs in that string.

Some characters have special meanings in regular expressions. For instance, a caret (^) at the beginning of a regular expression indicates that it must match the beginning of the string (or, more ...