From: Ammar Ali <ammarabuali@...>
Date: 2010-11-24T01:12:47+09:00
Subject: Re: Regex negative look-behind bug?

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On Tue, Nov 23, 2010 at 5:55 PM, Robert Klemme
<shortcutter@googlemail.com>wrote:

> On Tue, Nov 23, 2010 at 4:36 PM, Ammar Ali <ammarabuali@gmail.com> wrote:
> > On Tue, Nov 23, 2010 at 4:36 PM, Ruby Nuby <b1st@hotmail.com> wrote:
> >
> >> irb, Ruby 1.9.1
> >>
> >> What am I missing here?
> >>
> >> "b T T W b".match(/(?<!t t|a b) w/i)
> >> => nil
> >>
> >> #The second look-behind is now just a
> >> "b T T W b".match(/(?<!t t|a) w/i)
> >> => #<MatchData " W">
> >>
> >> #Regex stays the same, the T T are now in lower case
> >> "b t t W b".match(/(?<!t t|a) w/i)
> >> => nil
> >>
> >> #Look-behind only contains the t t condition now and, T T are back to
> >> upper case
> >> "b T T W b".match(/(?<!t t) w/i)
> >> => nil
> >
> > No bug here. It is doing exactly what you asked: only match a w if it is
> not
> > preceded by 't t'. In all cases the w is preceded by 't t', and in the
> case
> > that did match (?<!t t|a), the w was preceded by a 't t' but not an 'a',
> as
> > you asked, so it did match.
>
> That was an alternative!  If the RX in the lookbehind can match, the
> negative lookbehind must fail IMHO.
>
>
The thing is what's in the lookbehind, and all assertions for that matter,
is not really a regular expression. It is a fixed length literal. The only
exception, AFAIK, is character sets because they are also fixed length. The
engine needs to know how many characters to step back and examine.

Also the first alternative that matches wins. Here it is in lower case and
without ignoring case:

>> "b t t w b".match( /(?<!t t|a) w/ )
=> nil


There is a problem with the match though.  I suspect there is an issue
> with case sensitivity propagation
>
> irb(main):009:0> "b T T W b".match(/(?<!t t|a) w/i)
> => #<MatchData " W">
> irb(main):010:0> "b T T W b".match(/(?i:<!t t|a) w/i)
> => nil
>

That's not a valid assertion any more, it is now an options specification.

>> "b <!t t w b".match( /(?i:<!t t|a) w/ )
=> #<MatchData "<!t t w">



> irb(main):013:0> RUBY_VERSION
> => "1.9.1"
> irb(main):014:0> RUBY_PATCHLEVEL
> => 430
>

I initially tried the cases with 1.9.2, but I tried the above with the
latest 1.9.1 on my system (a bit older).

>> RUBY_VERSION
=> "1.9.1"
>> RUBY_PATCHLEVEL
=> 378

Regards,
Ammar

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