it might be an idea to try out both ANTLR and goldparser
and compare the results, I'd say the fastest one we'd take ;)
Michael Schmalle already created jasblocks, a Java port of his asblocks
which is able to parse AS3.
Since he's already created grammars for AS3, E4X, regex and ASDoc we might
be able to stand on the shoulders of giants :)
goldparser uses a different type of grammar notation, ANTLR uses EBNF and
uses BNF. I am not familiar with the differences but I'm guessing it should
to convert between the two without too much hassle.
After that its a matter of speed testing them.