Undocumented(?) limits on regexp functions

All the regexp functions blow up with "invalid memory alloc request"
errors when the input string exceeds 256MB in length. This restriction
does not seem to be documented anywhere that I could see.

(Also for regexp_split* and regexp_matches, there's a limit of 64M total
matches, which also doesn't seem to be documented anywhere).

Should these limits:

a) be removed

b) be documented

c) have better error messages?

-- 
Andrew (irc:RhodiumToad)