Custom Number Validation for WooCommerce Flexible Checkout Fields

fcf-custom-validation-number.php

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

/**
 * Function to validate the number
 *
 */
function wpdesk_fcf_validate_number( $field_label, $value ) {
    if ( ! ( ( is_numeric( $value ) ? intval( $value ) == $value : false ) ) ) {
        wc_add_notice( sprintf( '%s is not a valid number.', '<strong>' . $field_label . '</strong>' ), 'error' );
    }
}

add_filter( 'flexible_checkout_fields_custom_validation', 'wpdesk_fcf_custom_validation_number' );
/**
 * Add custom number validation
 *
 */
function wpdesk_fcf_custom_validation_number( $custom_validation ) {
    $custom_validation['number'] = array(
        'label'     => 'Number',
        'callback'  => 'wpdesk_fcf_validate_number'
    );

    return $custom_validation;
}

Dear Swoboda,

Do you have any suggestions to combine number validation with letters validation? in my country we use letters and numbers for bank accounts. I'm not that good at programming yet.

Kind regards