Custom field that supports validation by regular expressions

Following is a sample code snippet. To get the submitted field value, you can use $_POST["input_XXX"]

add_filter('gform_validation', 'custom_validation');
function custom_validation($validation_result){

    // set the form validation to false
    $validation_result["is_valid"] = false;
    $form = $validation_result["form"];

    // specify the first field to be invalid and provide custom validation message
    $form["fields"][0]["failed_validation"] = true;
    $form["fields"][0]["validation_message"] = "This field is invalid!";

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

    return $validation_result;

}