From: dblack@...
Date: 2006-01-07T12:49:32+09:00
Subject: Re: Oniguruma lookbehind question

Hi --

On Sat, 7 Jan 2006, K.Kosako wrote:

> dblack@wobblini.net wrote:
>
>> 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.
>
> Is this pattern work for you?
>
> str =  %Q{abc def "ghi jkl" mno}
> re3 = /((?<=")[^"]+(?="))|([\S&&[^"]]+)/
> p str.scan(re3)     #=> [[nil, "abc"], [nil, "def"], ["ghi jkl", nil], [nil, 
> "mno"]]

No; I get this:

[[nil, "\""], ["ghi jkl", nil], [nil, "\""]]

This:

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

gives me the result you got for your re3.  But it still seems to be
based on the right-hand alternate being checked first.


David

-- 
David A. Black
dblack@wobblini.net

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