XPath regular expression syntax, crippled?

Hi, I am so glad that Saxon 8.1.1 seems to do everything very nicely
now that I previously had trouble using multiple prior Saxon versions.

All except it implements the precise XSLT Regex standard. And I am
pulling my hair out, I can't seem to find a concise specification of
what that is, or is it really so heavily crippled as it appears?

XML Schema datatypes remove all the special parenthesis constructs
such as (?i:...), (?=...) or (?!...). Then XPath doesn't really
add most of it back either. Only the capturing groups, but no 
mention of non-capturing groups or positive or negative lookahead.
Major bummer!

So, what am I missing? If I am missing nothing, then I will quickly
ask Michael to consider a flag to Saxon that permits full
JDK 1.4+ regex syntax, or perhaps that he might allow doing this 
with a flag. It's amazing that Michael did all this work with that
regex translator machine only in order to cripple perfectly fine 
and useful regular expressions. Will there be any chance in working
on XSLT editors to get the full thing back?

regards,
-Gunther


-- 
Gunther Schadow, M.D., Ph.D.                    gschadow@xxxxxxxxxxxxxxx
Medical Information Scientist      Regenstrief Institute for Health Care
Adjunct Assistant Professor        Indiana University School of Medicine
tel:1(317)630-7960                         http://aurora.regenstrief.org