Custom validation for a number format

<?php
// http://www.gravityhelp.com/forums/topic/number-masked-field-keeps-resetting-tablet-keyboard-to-alpha
// change 160 here to your form ID
add_filter('gform_validation_160', 'force_input_format');
function force_input_format($validation_result){
        // this assumes the FHA Case # is entered in field one on your form
        // change this input_ number if it's a different field
        // try to match the first 10 characters after removing leading whitespace
        $case_number = substr(trim(rgpost('input_1')), 0, 10);
        // return a validation error if $case_number is not in the proper format, or if we needed to edit the $case_number at all before checking it.
        if (!preg_match("/\d{3}-\d{6}/", $case_number) || ($case_number != rgpost('input_1'))) {
        $validation_result['is_valid'] = false;
        foreach($validation_result['form']['fields'] as &$field){
            // field 1 is the field where we want to show the validation message
            if($field['id'] == 1){
                $field['failed_validation'] = true;
                                $field['validation_message'] = 'The FHA Case Number code you entered is invalid: please check the number and try again.';
                break;
            }
        }
    }
    return $validation_result;
}