title: AWS CloudFormation template validation when creating a Stack
body: <p>I'm creating an AWS Stack with a CF template in which I have a InstanceHostname that is limited to have max. 15 chars (normal letters and digits) using a regex in the &quot;AllowedPattern&quot;:</p>
  <pre><code>&quot;InstanceHostname&quot;: {
    &quot;Type&quot;: &quot;String&quot;,
    &quot;AllowedPattern&quot;: &quot;^((?![0-9]{1,15}$)[a-zA-Z0-9-]{1,15}$)?$&quot;,
    &quot;ConstraintDescription&quot;: &quot;The EC2 instance hostname must be 15 characters or less and must not...&quot;,
    &quot;Description&quot;: &quot;A custom name for the EC2 instance….&quot;
  },
  </code></pre>
  <p>Is there a way to have the &quot;AllowedPattern&quot; regex validation trigger, when entering the hostname on the &quot;Specify stack details&quot; page? As it is now the validation does not take place until selecting &quot;Create stack&quot; a couple of pages ahead.</p>