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Apache Struts 2 Documentation
How to validate field formats, such as a phone number
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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.

    <validators>
        <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>
            </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".