title: Regular expression validating html tags
body: <p>I am trying to do one validation using regular expression, but still not getting success. </p>
  
  <p>I need to validate on of my input which may be blank/empty. but If user has entered some value in it, </p>
  
  <p>it must be validate following validations.</p>
  
  <pre><code>1. maximum 20 characters allows.
  2. allowed [a-zA-Z0-9], but first character must be a latter (e.g. A-Z (any case))
  3. do not allowed html tags.
  </code></pre>
  
  <p>thanks.</p>