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//////////////////////////////////////////////////////////////
///////// CF7 FORMS PHONE VALIDATION
//////////////////////////////////////////////////////////////
add_filter( 'wpcf7_validate_email*', 'custom_phone_validation_filter', 20, 2 );
 
function custom_phone_validation_filter( $result, $tag ) {
    $tag = new WPCF7_FormTag( $tag );
    if ( 'phone' == $tag->name ) {
        $phone_num = isset( $_POST['phone'] ) ? trim( $_POST['phone'] ) : '';
        if(!preg_match("/^([+]?\d{1,4}[-\s]?|)\d{3}[-\s]?\d{3}[-\s]?\d{4}$/",$phone_num)) { 
            $result->invalidate( $tag, "Fornire un numero di telefono valido" );
        }
    }
    return $result;
}