title: How to use excel text function "NUMBERVALUE" from Apache POI formula
body: <p>I am using Apache POI to evaluate a formula cell that has a text function &quot;NUMBERVALUE&quot;, then I got an exception
  <em><strong>Caused by: org.apache.poi.ss.formula.eval.NotImplementedFunctionException: _xlfn.NUMBERVALUE</strong></em></p>
  <p>Interesting enough, when I try to regist this function  <code>WorkbookEvaluator.registerFunction(&quot;NUMBERVALUE&quot;,  new NumberRValue());</code>
  it gives me another error:
  <strong>java.lang.IllegalArgumentException: NUMBERVALUE is not a function from the Excel Analysis Toolpack.</strong></p>
  <p>I also tried to use the user-defined functions by implementing FreeRefFunction, it backs to the first error again.</p>
  <p>Caused by: org.apache.poi.ss.formula.eval.NotImplementedFunctionException: _xlfn.NUMBERVALUE</p>
  <p><strong>How to implement a text function in Apache POI?</strong></p>