Whereas the String and StringBuilder classes provide a set of methods that can be used to process string-based data, the RegEx and its supporting classes provide much more power for string-processing tasks. String processing mostly involves looking for patterns in strings (pattern matching) and it is performed via a special language called a regular expression. In this chapter we look at how to form regular expressions and how to use them to solve common text-processing tasks.

REGULAR EXPRESSIONS

A regular expression is a language that describes patterns of characters in strings, along with descriptors for repeating characters, alternatives, and groupings of characters. Regular expressions can be used to both perform searches in strings and perform substitutions in strings.

A regular expression itself consists of just a string of characters that define a pattern you want to search for in another string. Generally, the characters in a regular expression match themselves, so that the regular expression “the” matches that sequence of characters wherever they are found in a string.

A regular expression can also include special characters called metacharacters. Metacharacters are used to signify repetition, alternation, or grouping. We will examine how these metacharacters are used later in the chapter.

Most experienced computer users have used regular expressions in their work, even if they weren't aware they were doing so at the time. Whenever you type the following command at a command prompt:

1. C:\>dir myfile.exe

you are using the regular expression “myfile.exe”.