Introduction
Regular expressions are one of those quirky features that popup in a variety of programming languages, but because they seem to be a difficult concept to grasp, many developers push them away into the corner, forgetting that they even exist.
A regular expression is a specially formatted pattern that can be used to find instances of one string in another. Several programming languages including Visual Basic, Perl, JavaScript and PHP support regular expressions, and hopefully by the end of this primer you should be able to implement some basic regular expression functionality into your PHP pages.
Let's start by taking a look at what a regular expression is, and why you'd want to use them in your PHP pages.
What is a regular expression?
What do you think it is that separates programs like BBEdit and notepad from the good old console-based text editors? Both support text input and let you save that text into a file, however modern text editors also support other functionality including find-replace tools, which makes editing a text file that much easier.
Regular expressions are similar, only better. Think of a regular expression as an extremely advanced find-replace tool that saves us the pain of having to write custom data validation routines to check e-mail addresses, make sure phone numbers are in the correct format, etc.
One of the most common functions of any program is data validation, and PHP comes bundled with several text validation functions that allow us to match a string using regular expressions, making sure there's a space here, a question mark there, etc.
What you may not know however, is that regular expressions are simple to implement, and once you've mastered a few regular expressions (which are specially formatted strings that we can use to tell the regular expression engine the portion of a string we want to match) you'll be asking yourself why you left regular expressions in the corner for so long.

Note: PHP has two sets of functions for dealing with the two types of regular expression patterns: Perl 5 compatible patterns, and Posix standard compatible patterns. In this article we will be looking at the ereg function and working with search expressions that conform to the Posix standard. Although they don't offer as much power as Perl 5 patterns, they're a great way to start learning regular expressions. If you're interested in PHP's support for Perl 5 compatible regular expressions, then see the PHP.net site for details on the preg set of PHP functions.

PHP has six functions that work with regular expressions. They all take a regular expression string as their first argument, and are shown below:
This article originally appeared on WebReference.com.