title: What is wrong with my validation attribute derived from RegularExpressionAttribute?
body: <p>My attribute looks like this. To my mind, it is a <code>RegularExpressionAttribute</code>, but just automatically gets constructed with a fixed pattern and has a more user friendly <code>ErrorMessage</code> set.</p>
  
  <pre><code>public class NumericOnlyAttribute : RegularExpressionAttribute
  {
      public NumericOnlyAttribute()
          : base(@"\d+")
      {
          ErrorMessage = "Field must only contain numbers";
      }
  }
  </code></pre>