isn't the mongrel/thin/unicorn http parser library built using ragel (i.e. an existing parse library)?
Could you expand on why you think it's not possible to handle http with existing tools?
many parsers are written as automata, but that does mean it is in the same category as parsing tools such as LR, LL, GLR, PEG or CFG based approaches.
in http, handling things like 'Content-Length: %d' and then reading a subsequent length is a little harder. as is handling transfer-chunked encoding. http is quite fiendish in places :-)
these are 'data dependent' - the parse stream contains information (i.e length) on the subsequent tokens - although some regexps have back references, these aren't common place in parsing tools/formalisms like LR,LL,LALR,CFG or PEGs.
my point is simply that a lot of the parsing drama of late has revolved around the simple task of parsing a language, rather than parsing network protocols.
there is a larger class of parsing problems that are still to be tackled.