[xacml] regular expressions - subgroup follow-up

regular expressions are acknowledged as being tied to the POSIX (IEEE) 
standard. i suggest that we consider basic regular expresssions (BRE) as 
defined in POSIX specification 2:

http://www.opengroup.org/onlinepubs/007908799/xbd/re.html#tag_007_003

there is also an exteded regular expression set:

http://www.opengroup.org/onlinepubs/007908799/xbd/re.html#tag_007_004

have not really looked through it to discover the differences.

b