Regular expressions are a powerful tool for finding, examining and/or modifying text. Regular expressions themselves are, with a general pattern notation almost like a mini programming language, allowing you to define and parse text. They enable you to search for patterns within a string, extracting matches flexible and precise. However, you should note that because regular expressions are more powerful, they also suffer from added overhead and are slower than the more basic string functions. You should make careful consideration and only use regular expressions if you have a particular need.

PHP supports two different types of regular expressions: POSIX-extended and Perl-Compatible Regular Expressions (PCRE). The PCRE functions are more commonly used, are more powerful than the POSIX ones and faster as well

In a regular expression, most characters match only themselves. For instance, if you search for the regular expression “foo” in the string “only a fool does not use regular expressions” you get a match because “foo” occurs in that string. Some characters have a special meaning. For instance, the dollar sign ($) is used to match strings that end with the given pattern. Similarly, a caret (^) character at the beginning of a regular expression indicates that it must match the beginning of the string. Characters that match themselves are called literals while characters that have special meanings are called metacharacters. Read more »