GNU classpath from ikvm 0.24.0.1
A regular expression with a zero-width positive lookbehind does not compile:
Pattern x = Pattern.compile("(?<=X)") ;
It seems that lookbehind isn't supported by the regex implementation.
Similarly, zero-width negative lookbehind "(?<!X)"
This bug is the same as Bug #25812, and it has been fixed. As far as I know, IKVM uses a precompiled GNU Classpath library. But this fix was done only recently, and it is not reflected in IKVM.
*** This bug has been marked as a duplicate of 25812 ***