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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.

    <field name="phone">
        <field-validator type="regex">
            <param name="expression">\([\d][\d][\d]\) [\d][\d][\d]-[\d][\d][\d][\d]</param>
            <message>Phone number must be in the format (XXX) XXX-XXXX</message>

If your expression tests against alpha characters, you may be interested in the "caseSensitive" parameter of with Validator as well. It defaults to "true".