The Regular Expression Validator is one of the Validations that you can add to a Column defined in the Database Repository.
This validation gives you the flexibility and power of regular expressions to define patterns that are to be matched by column values.
See also
Use the Email Validator for email address fields, this is in its implementation a specialized Regular Expression Validator. This validator was added for convenience as it is needed often.
Read the .NET Framework Regular Expressions chapter (for version 2.0) on msdn to learn more about Microsoft's implementation of regular expressions in the .NET Framework 2.0.
More tutorials and examples on the use of regular expressions can be found on websites such as: http://www.regular-expressions.info and http://regexlib.com. The Wikipedia Programming language topics also cover it.