title: Regex to match backslash inside a string
body: <p>I'm trying to match the following strings:</p>
  
  <ul>
  <li><code>this\test_</code></li>
  <li><code>_thistes\t</code></li>
  <li><code>_t\histest</code></li>
  </ul>
  
  <p>In other words, the allowed strings have ONLY a backslash, splitting 2 substrings which can contain numbers, letters and _ characters.</p>
  
  <p>I tried the following regex, testing it on <a href="http://regexhero.net/tester/" rel="nofollow">http://regexhero.net/tester/</a>:
  <code>^[a-zA-Z_][\\\]?[a-zA-Z0-9_]+$</code></p>
  
  <p>Unfortunately, it recognizes also the following not allowed strings:</p>
  
  <ul>
  <li><code>this\\</code></li>
  <li><code>_\</code></li>
  <li><code>_\w\s\x</code></li>
  </ul>
  
  <p>Any help please?</p>