Programming C#, Third Edition

Regular Expressions

Regular expressions are a powerful language for describing and manipulating text. A regular expression is applied to a string—that is, to a set of characters. Often that string is an entire text document.

The result of applying a regular expression to a string is either to return a substring, or to return a new string representing a modification of some part of the original string. Remember that strings are immutable and so cannot be changed by the regular expression.

By applying a properly constructed regular expression to the following string:

One,Two,Three Liberty Associates, Inc.

you can return any or all of its substrings (e.g., Liberty or One), or modified versions of its substrings (e.g., LIBeRtY or OnE). What the regular expression does is determined by the syntax of the regular expression itself.

A regular expression consists of two types of characters: literals and metacharacters. A literal is a character you wish to match in the target string. A metacharacter is a special symbol that acts as a command to the regular expression parser. The parser is the engine responsible for understanding the regular expression. For example, if you create a regular expression:

^(From|To|Subject|Date):

this will match any substring with the letters "From,” "To,” "Subject,” or "Date,” so long as those letters start a new line (^) and end with a colon (:).

The caret (^) in this case indicates to the regular expression parser that the string you’re searching for must begin ...