Regular expressions are a powerful tool for examining and modifying text. Regular expressions themselves, with a general pattern notation almost like a mini programming language, allow you to describe and parse text. They enable you to search for patterns within a string, extracting matches flexibly and precisely. However, you should note that because regular expressions are more powerful, they are also slower than the more basic string functions. You should only use regular expressions if you have a particular need.
This tutorial gives a brief overview of basic regular expression syntax and then considers the functions that PHP provides for working with regular expressions.
PHP supports two different types of regular expressions: POSIX-extended and Perl-Compatible Regular Expressions (PCRE). The PCRE functions are more powerful than the POSIX ones, and faster too, so we will concentrate on them.