title: Flutter: how to avoid special characters in validator
body: <p>I have this validation function:</p>
  
  <pre><code>class FormFieldValidator{
    static String validate(String value, String message){
      return (value.isEmpty || (value.contains(**SPECIAL CHARACTERS**))) ? message : null;
    }
  }
  </code></pre>
  
  <p>I would like to indicate that doesn't have to contain special characters, but how can I say it?</p>