Method test

/**
 * Method to test if the file path is valid
 *
 * @param   \SimpleXMLElement  $element  The SimpleXMLElement object representing the `<field>` tag for the form field object.
 * @param   mixed              $value    The form field value to validate.
 * @param   string             $group    The field name group control value. This acts as an array container for the field.
 *                                       For example if the field has name="foo" and the group value is set to "bar" then the
 *                                       full field name would end up being "bar[foo]".
 * @param   Registry           $input    An optional Registry object with the entire data set to validate against the entire form.
 * @param   Form               $form     The form object for which the field is being tested.
 *
 * @return  boolean  True if the value is valid, false otherwise.
 *
 * @since   4.0.0
 */
public function test(\SimpleXMLElement $element, $value, $group = null, Registry $input = null, Form $form = null)
{
    // If the field is empty and not required, the field is valid.
    $required = (string) $element['required'] === 'true' || (string) $element['required'] === 'required';
    if (!$required && empty($value) && $value !== '0') {
        return true;
    }
    // Make sure we allow multiple classes to be added
    $cssIdentifiers = explode(' ', $value);
    foreach ($cssIdentifiers as $i => $identifier) {
        /**
         * The following regex rules are based on the Html::cleanCssIdentifier method from Drupal
         * https://github.com/drupal/drupal/blob/8.8.5/core/lib/Drupal/Component/Utility/Html.php#L116-L130
         *
         * with the addition for Joomla that we allow the colon (U+003A).
         */
        /**
         * Valid characters in a CSS identifier are:
         * - the hyphen (U+002D)
         * - a-z (U+0030 - U+0039)
         * - A-Z (U+0041 - U+005A)
         * - the underscore (U+005F)
         * - the colon (U+003A)
         * - 0-9 (U+0061 - U+007A)
         * - ISO 10646 characters U+00A1 and higher
         */
        if (preg_match('/[^\\x{002D}\\x{0030}-\\x{0039}\\x{0041}-\\x{005A}\\x{005F}\\x{003A}\\x{0061}-\\x{007A}\\x{00A1}-\\x{FFFF}]/u', $identifier)) {
            return false;
        }
        /**
         * Full identifiers cannot start with a digit, two hyphens, or a hyphen followed by a digit.
         */
        if (preg_match('/^[0-9]/', $identifier) || preg_match('/^(-[0-9])|^(--)/', $identifier)) {
            return false;
        }
    }
    return true;
}