It's not just Unicode either. I just mentioned EBCDIC because that particular regex has bit me before when I was translating perl scripts from Linux to zOS USS. Take a look at the code page for EBCDIC, you'll see quickly why it's a massive pain to sort through regexes like that.
I'm sure you've heard of the IBM AS/400 which is still firmly entrenched in MANY Fortune 500 companies. Not to mention tons of state and county government installations handling payroll, inventory, taxrolls, etc. I had to deal with a Perl script which dealt with ASCII to EBCDIC to port data to an Oracle database. If you're a Windows only shop, that's fine, but don't assume that anyone whom isn't is ancient.
