Custom Min/Max Character Limit Validation for WooCommerce Flexible Checkout Fields

fcf-custom-validation-length.php

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

/**
 * Function to validate the length
 *
 */
function wpdesk_fcf_validate_length( $field_label, $value ) {
    $min_length = 3;
    $max_length = 20;
    $value_length = strlen( $value );

    if ( $value_length < $min_length || $value_length > $max_length ) {
        wc_add_notice( sprintf( '%s is not valid. The length must be between %s and %s characters.', '<strong>' . $field_label . '</strong>', $min_length, $max_length ), 'error' );
    }
}

add_filter( 'flexible_checkout_fields_custom_validation', 'wpdesk_fcf_custom_validation_length' );
/**
 * Add custom length validation
 *
 */
function wpdesk_fcf_custom_validation_length( $custom_validation ) {
    $custom_validation['length'] = array(
        'label'     => 'Length',
        'callback'  => 'wpdesk_fcf_validate_length'
    );

    return $custom_validation;
}