Learning C# 3.0

Regular Expressions

As noted earlier, regular expressions provide a very powerful way to describe and manipulate text through pattern matching.

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 string objects 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 (such as “Liberty” or “One”) or modified versions of its substrings (such as “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 want 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 of the following substrings: From, To, Subject, or Date, as long as the substring starts a new line (^) and ends with a colon (:).

The caret (^) indicates to the regular expression parser that the string you’re searching for must begin a new line. The substrings From and To are literals, and the metacharacters left and ...