There is two reasons for this limitation. First reason is mentioned by David. There is no syntax to backreference a group with number > 99 (but there is a syntax for conditional groups and for substitutions). Second reason is that current implementation of regexp engine uses an array of constant size for groups.

Here is a patch which increases static limit to 1000 groups. It also allows to specify arbitrary group number in form of "(?P=number)". This is conformed to the syntax of conditional groups and for substitutions.