title: Syntax error in selenium xpath expression using regex
body: <p>In Selenium 2.42.2 and Firefox 29,  what is wrong with this XPath expression using regex:</p>
  
  <pre><code>//button[matches(text(),'\s*ABC\s*')]
  </code></pre>
  
  <p>It gives the following error message:</p>
  
  <pre><code>[Exception... "The expression is not a legal expression."  code: "12" nsresult: "0x805b0033 (SyntaxError)"  location: "&lt;unknown&gt;"]
  </code></pre>