title: How to write custom RegularExpressionValidator which takes the values from the config file?
body: <p>I have to use a Regular expression validator for username property in my model.
  I am getting this regular expression from the config file.</p>
  
  <pre><code>[RegularExpression(UsernameValidationExpression)] //UsernameValidationExpression = value from the config file
  public string UserName { get; set; }
  </code></pre>
  
  <p>Here i am getting an error 
  "An attribute argument must be a constant expression, typeof expression or array creation expression of an attribute parameter type"</p>
  
  <p>How can i solve this??</p>