Validation to force matching fields

<?php
// http://www.gravityhelp.com/forums/topic/field-validation
// http://www.gravityhelp.com/forums/topic/match-fields
// update the 21 here to your form ID
add_filter('gform_validation_21', 'matching_field_validation', 10, 2);
function matching_field_validation($validation_result){

    $form = $validation_result['form'];
    // this compares field 1 to field 2, update these values to match your form
    if (rgpost('input_1') !== rgpost('input_2')) {
        // this is the validation error for input_1
        $form['fields'][0]['failed_validation'] = true;
        $form['fields'][0]['validation_message'] = 'This field must match the next one.';
        // this is the validation error for input_2
        $form['fields'][1]['failed_validation'] = true;
        $form['fields'][1]['validation_message'] = 'Please verify your entries.';
        $validation_result['is_valid'] = false;
    }

    // update the form in the validation result with the form object you modified
    $validation_result['form'] = $form;
    return $validation_result;
}