title: preg_match for matching strings like "max_length[:num:]"
body: <p>I have some strings like <code>max_length[:num:]</code>where <code>:num:</code> can be any number ie(<code>max_length[50]</code> or <code>max_length[100]</code> etc).</p>
  
  <p>What will be the preg_match code for such strings in PHP?</p>