invalid XPath syntax leads to RuntimeException

[Tahura Chaudhry]

In my application, I provide a UI to allow the user to enter an XPath 
expression.  I call Xalan's XPathEvaluator.evaluate(....) to determine if 
they syntax of the entered expression is valid.  If evalute() throws a 
DOMException or an XPathException, I assume that the XPath syntax is invalid 
and notify the user.  I also display the exception string 
(exception.getMessage()) to the user in the UI.

This works well for most cases. However I have come across a case in which 
entering an invalid syntax leads to a java.lang.RuntimeException.

The XPath expression that causes this is:
      [EMAIL PROTECTED]"manager"] and
I can't assume that the RuntimeException will always be thrown because of 
invalid syntax.

Is there a better way to validate the syntax of the XPath expression?
The entire exception trace resulting from calling evalute() on the above 
XPath expression is shown below:

    [java] java.lang.RuntimeException: Programmer's assertion in 
getNextStepPos: unknown stepType:
-1
    [java] java.lang.RuntimeException: Programmer's assertion in 
getNextStepPos: unknown stepType:
-1
    [java]     at 
org.apache.xpath.compiler.OpMap.getNextStepPos(OpMap.java:278)
    [java]     at 
org.apache.xpath.axes.DescendantIterator.<init>(DescendantIterator.java:119)
    [java]     at 
org.apache.xpath.axes.WalkerFactory.newDTMIterator(WalkerFactory.java:298)
    [java]     at 
org.apache.xpath.compiler.Compiler.locationPath(Compiler.java:670)
    [java]     at 
org.apache.xpath.compiler.Compiler.compile(Compiler.java:210)
    [java]     at 
org.apache.xpath.compiler.Compiler.compileOperation(Compiler.java:248)
    [java]     at org.apache.xpath.compiler.Compiler.or(Compiler.java:285)
    [java]     at 
org.apache.xpath.compiler.Compiler.compile(Compiler.java:158)
    [java]     at 
org.apache.xpath.compiler.Compiler.compile(Compiler.java:156)
    [java]     at org.apache.xpath.XPath.<init>(XPath.java:209)
    [java]     at org.apache.xpath.XPath.<init>(XPath.java:238)
    [java]     at 
org.apache.xpath.domapi.XPathEvaluatorImpl.createExpression(XPathEvaluatorImpl.ja
va:165)
    [java]     at 
org.apache.xpath.domapi.XPathEvaluatorImpl.evaluate(XPathEvaluatorImpl.java:257)