Programming C# 3.0, 5th 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 one of the following:

• To find out whether the string matches the regular expression

• To return a substring

• To return a new string representing a modification of some part of the original string

(Remember that strings are immutable, and so can't 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, as long as those letters start a new line (^) and end with a colon (:).

The caret (^) in this case indicates to the regular expression ...