Section 13.0. Introduction

13.0. Introduction

One of the most powerful features added to ActionScript 3.0 is regular expressions (more commonly known as regexes or regexps). Regular expressions are, put simply, patterns that can be matched against strings. You may be familiar with other types of patterns, such as wildcards (e.g., * and ?), which can be used to match patterns while searching for files. Patterns are also used in Recipe 9.5. Regular expressions support this type of pattern matching, but they are also much more sophisticated.

Regular expressions can be useful in many situations. For instance, the patterns can be applied against strings to perform a variety of tasks, including:

• Finding substrings beginning with a vowel (a, e, i, o, or u)

• Extracting specific values within a string, such as the year value from a full date

• Validating user input to ensure an email address is formatted correctly

• Stripping out HTML tags from a block of text to remove the markup

The patterns used for regular expressions are built by combining characters that have special meaning and can range from being very simple:

[a-zA-Z]

to being extremely complex and cryptic, such as this regex for matching a valid IP address:

^([01]?\d\d?|2[0-4]\d|25[0-5])\.([01]?\d\d?|2[0-4]\d|25[0-5])\.([01]?\d\d?|2[0-4]\d|25[0-5])\.([01]?\d\d?|2[0-4]\d|25[0-5])$

Simple patterns, such as .*, are easy to understand, but more complex patterns are difficult to learn and are even harder to implement. Thankfully, every regular expression can be broken down into a plain English description. For example, the simple regular expression .* means "any character repeated any number of times." More complex patterns, such as (A|a)ction(S|s)cript, are no different because they are built by combining simple patterns in various ways. The pattern (A|a)ction(S|s)cript means "either a capital or lowercase 'a', followed by the string 'ction', followed by either a capital or lowercase 's', followed by the string 'cript'," and can be used to find occurrences of "ActionScript" (and subtle variations, such as "actionScript") in a string.

Mastering the syntax of regular expressions is not trivial. As with anything else, a little effort to learn the basic principles can go a long way, and knowing and understanding regular expressions is a powerful tool to have at your fingertips.

To learn more about regular expressions, you should get a copy of Mastering Regular Expressions, by Jeffrey E.F. Friedl (O'Reilly), or Regular Expressions Pocket Reference, by Tony Stubblebine (O'Reilly). Both books teach you everything you need to know about regexes.

Despite how complex regular expression patterns can be, it's fairly easy to use regular expressions in ActionScript, as you'll soon discover. This chapter focuses on the more common uses of regular expressions within the context of Flash and ActionScript, and by no means is an exhaustive or comprehensive guide.