title: if else failing regex
body: <p>I want to write a regex to find if word is of 3 characters length, but preceding by <code>m_</code> is optional. In that case <code>m_</code> followed by minimum of 3 characters is required.</p>
  
  <p>Basically I want to match</p>
  
  <p><code>Abc</code> or <code>m_abc</code> and dont match <code>ab</code> or <code>m_ac</code></p>
  
  <pre><code>(^(m_))?([a-zA-Z0-9_]{3,})|(^[a-zA-Z0-9_]{3,}$)
  </code></pre>
  
  <p>I tried an if loop but it is matching the text <code>m_a</code> also.
  Can you please help me what I am missing here</p>
  
  <p>Maybe I wrote my regex wrong. 
  I want something like
  if(m_ found)
       "followed by 3 characters required"
  else
       "Look if total number of characters is 3"</p>
  
  <p>Thanks.</p>