Caleb Clausen
2/25/2008 10:12:00 PM
On 2/23/08, makoto kuwata <kwa@kuwata-lab.com> wrote:
> Hi,
>
> I'm planning to implement StringScanner in pure Ruby.
> But I found that it is hard to implement StringScanner#scan()
> in pure Ruby, because of the difference between Regexp#match()
> and StringScanner#scan().
>
> StringScanner#scan() matches only when pattern matches at the
> beginning (or at the current position) of input string.
>
> require 'strscan'
> input = 'foo 123'
> scanner = StringScanner.new(input)
> p scanner.scan(/\d+/) #=> nil
>
> But Regexp#match() matches whenever input string contains pattern.
>
> input = 'foo 123'
> m = /\d+/.match(input)
> p m[0] if m #=> "123"
>
>
> Is it possible to restrict Regexp#match() to match only when
> pattern starts at the beginning of input string?
> My idea is to convert /regexp/ into /\A(?:regexp)/ every time,
> but it is a litte ugly.
> Is there any good idea to emulate StringScanner#scan in pure Ruby?
I've done this kind of thing before, and rewriting the regex was the
best I could come up with. If Michael's suggestion is too slow for
you, then regex rewriting is the only game in town. (Actually, if you
can determine that your regexes always require some leading substring,
you might be able to optimize Michael's way a bit more...)
If speed is an issue, why not just use the existing StringScanner?
Creating regex's at runtime can cost you quite a bit in performance as
well... some caching can help here, if the same regexes are likely to
be encountered again.
You might want to take a look at String#index (and it's 2nd parameter)
rather than String#match or Regexp#match, as it allows you to start
matching wherever you want in the string, rather than just the
beginning. That doesn't help with your immediate question, but maybe
it'll give you some ideas of different ways to approach it.
Finally, a moment of self-promotion. My library 'sequence' implements
basically what you want (using regex rewriting, which in the full
elaboration gets rather involved). Maybe it could save you some
effort...