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Chapter 8. Regular Expressions

Included in the .NET Framework Class Library is the System.Text.RegularExpressions namespace that is devoted to creating, executing, and obtaining results from regular expressions executed against a string.

Regular expressions take the form of a pattern that can be matched to zero or more characters within a string. The simplest of these patterns, such as .* (match anything and everything) and [A-Za-z] (match any letter) are easy to learn, but more advanced patterns can be difficult to learn and even more difficult to implement correctly. Learning and understanding regular expressions can take considerable time and effort, but the work will pay off.

Regular expression patterns can take a simple form—such as a single word or character—or a much more complex pattern. The more complex patterns can recognize and match such things as the year portion of a date, all of the <SCRIPT> tags in an ASP page, or a phrase in a sentence that varies with each use. The .NET regular expression classes provide a very flexible and powerful way to do such things as recognize text, replace text within a string, and split up text into individual sections based on one or more complex delimiters.

Despite the complexity of regular expression patterns, the regular expression classes in the FCL are easy to use in your applications. Executing a regular expression consists of the following steps:

Create an instance of the Regex object that contains the regular expression pattern along with any options for executing that pattern.
Retrieve a reference to an instance of the Match object by calling the Match instance method if you want only the first match found, or to an instance of the MatchesCollection object by calling the Matches instance method if you want more than just the first match found.
If you've called the Matches method to retrieve a MatchCollection object, iterate over the MatchCollection using a foreach loop. Each iteration will allow access to every Match object that the regular expression produced.

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