[LRUG] Parsing text

Roland Swingler roland.swingler at gmail.com
Wed May 14 02:36:25 PDT 2014

I think your two examples are quite different - one is a formal language
which you could definitely use something like a parser to handle whereas
the other sounds a lot more messy - I'm not sure you're going to be able to
do better than hacky stringing together regexps in that case.

I did a presentation on treetop back in the dark days of LRUG 2009 if
you're interested:
also, have a look at parslet:
http://kschiess.github.io/parslet/ if you're interested in treetop -
parslet has a lot nicer error reporting, and the benefit of being real ruby
than odd sort-of-ruby.


On Wed, May 14, 2014 at 8:49 AM, Andrew Stewart
<boss at airbladesoftware.com>wrote:

> Hello El Rug!
> From time to time I encounter a situation where I would like to parse
> (semi-)structured text.  I'm sure this is trivial with the correct
> approach.  Regrettably I don't know anything about parsers/compilers/etc
> and I end up hand-rolling fragile, line-based state machines which are soon
> impossible to reason about.
> I'd like to know how to do this properly but I don't know where to begin.
> Here are a couple of specific examples:
> - In 2006 before SASS etc existed, I wrote a Rails plugin for nested CSS.
>  It read a nested stylesheet and flattened it into normal CSS.  Back then I
> wasn't sure how to parse a nested stylesheet...and I still don't know how.
>  (Stop laughing at the back!)
> - A few months ago I needed to convert hospital admissions records from a
> PDF to CSV.  Each record had fields like id, name, various dates, clinical
> history, attending doctor, etc.  The fields weren't always in the same
> order due to the layout of text in the PDF, and some fields were optional.
>  Sometimes there were several fields on a line, and a field could be spread
> over several lines.  I did my usual thing of looping over each line,
> matching field names with regular expressions, and trying to keep track of
> where I was with a state variable.  Its sole virtue was that it (sort of)
> worked; otherwise it was horrible: hard to understand, hard to modify, hard
> to extend, and very hard to debug.
> Please could someone enlighten me?
> Cheers,
> Andy Stewart
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