Even though I still use regexes in rare circumstances - e.g. inside config files, parser combinators already do a much better job than this (or regexes) when you are writing maintainable code:

    warcEntry = do
        header <- warcHeader
        crlf
        body <- do
            contentLength <- getContentLength header
            compressionMode <- getCompressionMode header
            warcbody contentLength compressionMode
        crlf
        crlf
        return (WarcEntry header body)

If you accept crlf as "carriage-return-line-feed", the rest basically reads as pseudocode. crlf could have just as easily been written (string "\r\n") I guess.

Parser combinators can:

* call out to other parsing functions (e.g. warcHeader) - so you can build your code out of testable units.

* bind results to variables and start using them during the parse, e.g. warcHeader returns data containing contentLength and compressionMode, which is then fed to the warcbody function so it knows what to expect.