Password validation using regex.

password-regex.php

  /**
   * Password Validation
   * 
   * @link https://stackoverflow.com/a/21456918/5449906
   *
   * @param $string
   *
   * @return bool
   */
  private function validatePassword( $password )
  {
    if ( false ) {
      /** Minimum eight characters, at least one letter and one number: */
      $regex = "/^(?=.*[A-Za-z])(?=.*\d)[A-Za-z\d]{8,}/";

      /** Minimum eight characters, at least one letter, one number and one special character: */
      $regex = "/^(?=.*[a-z])(?=.*[A-Z])(?=.*\d)[a-zA-Z\d]{8,}$/";

      /** Minimum eight characters, at least one uppercase letter, one lowercase letter and one number: */
      $regex = "/^(?=.*[a-z])(?=.*[A-Z])(?=.*\d)(?=.*[@$!%*?&])[A-Za-z\d@$!%*?&]{8,}$/";

      /** Minimum eight and maximum 10 characters, at least one uppercase letter, one lowercase letter, one number and one special character: */
      $regex = "/^(?=.*[a-z])(?=.*[A-Z])(?=.*\d)(?=.*[@$!%*?&])[A-Za-z\d@$!%*?&]{8,10}$/";
    }
    else
    {
      return false;
    }
}

Non-functional.