Validating the format of String fields for patterns (such as a phone number) is easy with StringRegexValidator (named "regex" in the default validator configuration).
Simply add the validator the field in question, and supply a regular expression to match it against.
<validators> <field name="phone"> <field-validator type="regex"> <param name="regex">\([WW:\d][WW:\d][WW:\d]\) [WW:\d][WW:\d][WW:\d]-[WW:\d][WW:\d][WW:\d][WW:\d]</param> <message>Phone number must be in the format (XXX) XXX-XXXX</message> </field-validator> </field> </validators>
If your expression tests against alpha characters, you may be interested in the "caseSensitive" parameter of with Validator as well. It defaults to "true".