gistfile1.html

<xhtml:html xmlns:xhtml="http://www.w3.org/1999/xhtml"
            xmlns:xforms="http://www.w3.org/2002/xforms"
            xmlns:xxforms="http://orbeon.org/oxf/xml/xforms"
            xmlns:ev="http://www.w3.org/2001/xml-events"
            xmlns:xs="http://www.w3.org/2001/XMLSchema">
    <xhtml:head>
        <xforms:model>
            <xforms:instance>
                <form>
                    <sample>
                        <firstname/>
                        <lastname/>
                    </sample>
                </form>
            </xforms:instance>
            <xforms:bind ref="sample">
                <xforms:bind nodeset="firstname" type="xs:string" required="true()"
                             constraint="matches(.,'[A-Za-z]{6,15}')"/>
            </xforms:bind>
        </xforms:model>
    </xhtml:head>
    <xhtml:body>
        <xforms:input ref="sample/firstname">
            <xforms:alert>Incorrect value</xforms:alert>
        </xforms:input>
    </xhtml:body>
</xhtml:html>