PHP Cookbook, 3rd Edition

Chapter 23. Regular Expressions

Introduction

Regular expressions are an intricate and powerful tool for matching patterns and manipulating text. Though not as fast as plain-vanilla string matching, regular expressions are extremely flexible. They allow you to construct patterns to match almost any conceivable combination of characters with a simple—albeit terse and punctuation-studded—grammar. If your website relies on data feeds that come in text files—data feeds like sports scores, news articles, or frequently updated headlines—regular expressions can help you make sense of them.

This chapter gives a brief overview of basic regular expression syntax and then focuses on the functions that PHP provides for working with regular expressions. For a bit more detailed information about the ins and outs of regular expressions, check out the PCRE section of the PHP online manual and Appendix B of Learning PHP 5 by David Sklar (O’Reilly). To start on the path to regular expression wizardry, read the comprehensive Mastering Regular Expressions by Jeffrey E.F. Friedl (O’Reilly).

Regular expressions are handy when transforming plain text into HTML, and vice versa. Luckily, because these are such helpful subjects, PHP has many built-in functions to handle these tasks, explained by recipes in other chapters. Preventing Cross-Site Scripting tells how to escape HTML entities; Removing HTML and PHP Tags covers stripping HTML tags; Recipes and show how to convert plain text to HTML and HTML to plain ...