Custom URL Validation for WooCommerce Flexible Checkout Fields

fcf-custom-validation-url.php

<?php
// Do NOT include the opening php tag

/**
 * Function to validate the URL
 *
 */
function wpdesk_fcf_validate_url( $field_label, $value ) {
    if ( filter_var( $value, FILTER_VALIDATE_URL ) == false ) {
        wc_add_notice( sprintf( '%s is not a valid URL.', '<strong>' . $field_label . '</strong>' ), 'error' );
    }
}

add_filter( 'flexible_checkout_fields_custom_validation', 'wpdesk_fcf_custom_validation_url' );
/**
 * Add custom URL validation
 *
 */
function wpdesk_fcf_custom_validation_url( $custom_validation ) {
    $custom_validation['url'] = array(
        'label'     => 'URL',
        'callback'  => 'wpdesk_fcf_validate_url'
    );

    return $custom_validation;
}