From: makoto kuwata <kwa@...>
Date: 2008-02-24T13:20:08+09:00
Subject: [Q] difference between StringScanner#scan and Regexp#match

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?

--
regards,
makoto kuwata