title: Question mark with equal to sign in regex
body: <p>Hey Experts i am new to regex.I am really confused by studying this regex.I have found something which is very difficult to understand for me.The thing is the use of question mark and equal to symbol in regex.An eg :</p>
  
  <pre><code>"(?&lt;=\d)(\s)(?=[\d-])"
  </code></pre>
  
  <p>I just need to know the use of <code>?=</code> in this regex  code..I have searched google many times in this case but i didnt find any solution there.So i came here It will be a great help for me if you answer this one correctly for me  ..:) ..</p>
  
  <p>Thanks in advance ..</p>