Programming ASP.NET, 3rd Edition

Regular Expressions

Often, a simple value or range check is insufficient; you must check that the form of the data entered is correct. For example, you may need to ensure that a Zip Code is five digits, an email address is in the form [e-mail], a credit card matches the right format, and so forth.

A regular expression validator allows you to validate that a text field matches a regular expression. Regular expressions are a language for describing and manipulating text.

Tip

For complete coverage of regular expressions , see Mastering Regular Expressions, Second Edition, by Jeffrey Friedl (O’Reilly).

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.) Consider this regular expression:

^\d{5}$

This will match any string that has five numerals. The initial metacharacter, ^, indicates the beginning of the string. The second metacharacter, \d, indicates a digit. The third metacharacter, {5}, indicates five of the digits, and the final metacharacter, $, indicates the end of the string. Thus, this regular expression matches five digits between the beginning and end of the line and nothing else.

Tip

When you use a RegularExpressionValidator control with client-side validation, the regular expressions are matched using ...