title: Groovy Postbuild Plugin manager.getLogMatcher regex returns null if parentheses are present in string
body: <p>Jenkins Groovy Postbuild Plugin <code>manager.getLogMatcher</code> regex returns <code>null</code> if parentheses are present in string.</p>
  
  <p>Jenkins console output contains string:</p>
  
  <pre><code>(origin/pull-requests/1365/merge)
  </code></pre>
  
  <p>I need to get number from string 1365 and use:</p>
  
  <pre><code>manager.getLogMatcher("^origin/pull-requests/(.*?)/merge*").matches()
  </code></pre>
  
  <p>it fails with:</p>
  
  <pre><code>Java.lang.NullPointerException: Cannot invoke method matches() on null object
  </code></pre>
  
  <p>but if I delete parentheses:</p>
  
  <pre><code>origin/pull-requests/1365/merge
  </code></pre>
  
  <p>everything is ok.</p>