From: dblack@...
Date: 2006-01-07T11:56:50+09:00
Subject: Oniguruma lookbehind question

Hi --

For some reason, lookbehind and alternation seem not to be playing
together in a little Oniguruma test.  This is based on the string
splitting thread from a little while ago this evening, and uses a CVS
1.9.0 Ruby acquired about 1/2 an hour ago.

   str =  %Q{abc def "ghi jkl" mno}

   # Look for "..." but just get the ... part:
   re1 = /(?<=")[^"]+(?=")/

   # Test that:
   p str.scan(re1)  # => ["ghi jkl"]

   # Now, do the same thing *or* \S+.  This should, I think,
   # pick up the abc, def, and mno substrings too.

   re2 = /((?<=")[^"]+(?="))|(\S+)/

   # But it doesn't; the part before the alternation never
   # matches, even though it did before (as shown by the
   # captures):

   p str.scan(re2)
   # => [[nil, "abc"], [nil, "def"], [nil, "\"ghi"], [nil, "jkl\""],
   #     [nil, "mno"]]

I know that's all a bit cluttered, but the basic thing is that a
sub-pattern using lookbehind doesn't seem to match any more when
there's an alternation.  Instead, only the second alternative ever
matches.

Does anyone know why?


David

-- 
David A. Black
dblack@wobblini.net

"Ruby for Rails", from Manning Publications, coming April 2006!
http://www.manning.com/books/black