FULL PRODUCT VERSION :
java version "1.6.0_16"
Java(TM) SE Runtime Environment (build 1.6.0_16-b01)
Java HotSpot(TM) Client VM (build 14.2-b01, mixed mode, sharing)


ADDITIONAL OS VERSION INFORMATION :
Distributor ID: Ubuntu
  Description: Ubuntu 9.04
  Release: 9.04
Codename: jaunty


A DESCRIPTION OF THE PROBLEM :
When you try to evaluate an XPath expression with redundant unary '-' signs you get a parser error. Recursive Unary-Expressions are accepted by XPath 1.0 standard

http://www.w3.org/TR/xpath#NT-UnaryExpr
[27] UnaryExpr ::= UnionExpr
                                 | '-' UnaryExpr

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
javax.xml.xpath.XPathFactory.newInstance().newXPath().evaluate("--2",(Object)null)

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Should return "2"
ACTUAL -
javax.xml.transform.TransformerException: A location path was expected, but the following token was encountered: -


ERROR MESSAGES/STACK TRACES THAT OCCUR :
Exception in thread "main" javax.xml.transform.TransformerException: A location path was expected, but the following token was encountered: -
at com.sun.org.apache.xpath.internal.compiler.XPathParser.error(XPathParser.java:608)
at com.sun.org.apache.xpath.internal.compiler.XPathParser.LocationPath(XPathParser.java:1599)
at com.sun.org.apache.xpath.internal.compiler.XPathParser.PathExpr(XPathParser.java:1315)
at com.sun.org.apache.xpath.internal.compiler.XPathParser.UnionExpr(XPathParser.java:1234)
at com.sun.org.apache.xpath.internal.compiler.XPathParser.UnaryExpr(XPathParser.java:1140)
at com.sun.org.apache.xpath.internal.compiler.XPathParser.MultiplicativeExpr(XPathParser.java:1061)
at com.sun.org.apache.xpath.internal.compiler.XPathParser.AdditiveExpr(XPathParser.java:1003)
at com.sun.org.apache.xpath.internal.compiler.XPathParser.RelationalExpr(XPathParser.java:928)
at com.sun.org.apache.xpath.internal.compiler.XPathParser.EqualityExpr(XPathParser.java:868)
at com.sun.org.apache.xpath.internal.compiler.XPathParser.AndExpr(XPathParser.java:832)
at com.sun.org.apache.xpath.internal.compiler.XPathParser.OrExpr(XPathParser.java:805)
at com.sun.org.apache.xpath.internal.compiler.XPathParser.Expr(XPathParser.java:788)
at com.sun.org.apache.xpath.internal.compiler.XPathParser.initXPath(XPathParser.java:127)
at com.sun.org.apache.xpath.internal.XPath.<init>(XPath.java:176)
at com.sun.org.apache.xpath.internal.XPath.<init>(XPath.java:264)
at com.sun.org.apache.xpath.internal.jaxp.XPathImpl.eval(XPathImpl.java:193)
at com.sun.org.apache.xpath.internal.jaxp.XPathImpl.evaluate(XPathImpl.java:275)
at com.sun.org.apache.xpath.internal.jaxp.XPathImpl.evaluate(XPathImpl.java:365)
at oracle.bpm.project.compiler.test.Test.main(Test.java:19)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at com.intellij.rt.execution.application.AppMain.main(AppMain.java:90)
--------------- linked to ------------------
javax.xml.xpath.XPathExpressionException
at com.sun.org.apache.xpath.internal.jaxp.XPathImpl.evaluate(XPathImpl.java:289)
at com.sun.org.apache.xpath.internal.jaxp.XPathImpl.evaluate(XPathImpl.java:365)
at oracle.bpm.project.compiler.test.Test.main(Test.java:19)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at com.intellij.rt.execution.application.AppMain.main(AppMain.java:90)
Caused by: javax.xml.transform.TransformerException: A location path was expected, but the following token was encountered: -
at com.sun.org.apache.xpath.internal.compiler.XPathParser.error(XPathParser.java:608)
at com.sun.org.apache.xpath.internal.compiler.XPathParser.LocationPath(XPathParser.java:1599)
at com.sun.org.apache.xpath.internal.compiler.XPathParser.PathExpr(XPathParser.java:1315)
at com.sun.org.apache.xpath.internal.compiler.XPathParser.UnionExpr(XPathParser.java:1234)
at com.sun.org.apache.xpath.internal.compiler.XPathParser.UnaryExpr(XPathParser.java:1140)
at com.sun.org.apache.xpath.internal.compiler.XPathParser.MultiplicativeExpr(XPathParser.java:1061)
at com.sun.org.apache.xpath.internal.compiler.XPathParser.AdditiveExpr(XPathParser.java:1003)
at com.sun.org.apache.xpath.internal.compiler.XPathParser.RelationalExpr(XPathParser.java:928)
at com.sun.org.apache.xpath.internal.compiler.XPathParser.EqualityExpr(XPathParser.java:868)
at com.sun.org.apache.xpath.internal.compiler.XPathParser.AndExpr(XPathParser.java:832)
at com.sun.org.apache.xpath.internal.compiler.XPathParser.OrExpr(XPathParser.java:805)
at com.sun.org.apache.xpath.internal.compiler.XPathParser.Expr(XPathParser.java:788)
at com.sun.org.apache.xpath.internal.compiler.XPathParser.initXPath(XPathParser.java:127)
at com.sun.org.apache.xpath.internal.XPath.<init>(XPath.java:176)
at com.sun.org.apache.xpath.internal.XPath.<init>(XPath.java:264)
at com.sun.org.apache.xpath.internal.jaxp.XPathImpl.eval(XPathImpl.java:193)
at com.sun.org.apache.xpath.internal.jaxp.XPathImpl.evaluate(XPathImpl.java:275)
... 7 more


REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
import javax.xml.xpath.XPathFactory;

public class Test
{
    public static void main(String[] args) throws Exception
    {
        System.out.println(XPathFactory.newInstance().newXPath().evaluate("--2",(Object)null));
    }
}

---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
Do not use redundant "-" signs