If you mean a parser for a notation expressing regular grammars, then it only takes a couple of hours to knock out a recursive descent parser (if you know what you're doing).
I mean implementable by your standard regex library in your favorite programming language, which is largely a PCRE-style regular expression (although backreferences and lookahead/lookbehind aren't necessary).
Because they're generally not regular.
If you mean a parser for a notation expressing regular grammars, then it only takes a couple of hours to knock out a recursive descent parser (if you know what you're doing).