PHP snippet for Gravity Forms UK postcode validation using regex pattern in functions.php. Ensures proper UK postcode format on form submit.

gravity-forms-uk-postcode-validation.php

<?php
/**
 * UK Postcode Validation for Gravity Forms
 * Adds custom validation using regex to ensure proper UK postcode format.
 * 
 * Usage: Replace X_Y with your form ID and field ID (e.g., 5_3).
 *
 * @author Faisal Ahammad <[email protected]>
 * @source https://community.gravityforms.com/t/input-mask-for-uk-postcode/20004
 */

add_filter( 'gform_field_validation_X_Y', 'validate_uk_postcode', 10, 4 );
function validate_uk_postcode( $result, $value, $form, $field ) {
    
    if ( $result['is_valid'] ) {
        
        // UK postcode regex pattern supporting formats with or without space
        $pattern = '/^([A-Z]{1,2}\d{1,2}[A-Z]?)\s?(\d[A-Z]{2})$/i';
        
        $clean_value = trim( preg_replace( '/\s+/', ' ', $value ) );
        
        // Validate postcode format
        if ( ! preg_match( $pattern, $clean_value ) ) {
            $result['is_valid'] = false;
            $result['message'] = 'Please enter a valid UK postcode (e.g., SW1A 1AA).';
        }
    }
    
    return $result;
}