title: Is there a grammar for Maven/Plexus property expression syntax
body: <p>I've been looking for documentation on the maven expression syntax.</p>
  
  <p>I managed to find the <a href="https://github.com/codehaus-plexus/plexus-interpolation/" rel="nofollow">plexus-interpolation module</a> which seems to do the work, but its documentation is pretty sparse.</p>
  
  <p>Is there any documentation on the expression format better than the <a href="https://github.com/codehaus-plexus/plexus-interpolation/blob/master/src/test/java/org/codehaus/plexus/interpolation/StringSearchInterpolatorTest.java" rel="nofollow">unittests</a>?</p>
  
  <p><a href="https://codehaus-plexus.github.io/plexus-interpolation/" rel="nofollow">https://codehaus-plexus.github.io/plexus-interpolation/</a> has some usage information, which suggests that the Regex based interpolator as the preferred one.</p>