Forum: Ruby Help with regular expression

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B2896675ee61d866e6a5a19407730602?d=identicon&s=25 toulax@gmail.com (Guest)
on 2007-06-28 20:56
(Received via mailing list)
How can I make a regular expression that will match everything, unless
it contains a certain string then it will match nothing.

For instance, let's say it should not match "bar", then:

"foo" => "foo"
"bar " => nil
"foobar" => nil

Thanks in advance
48124635945b45221ba12a26371f9e3e?d=identicon&s=25 Philip Hallstrom (Guest)
on 2007-06-28 21:01
(Received via mailing list)
> How can I make a regular expression that will match everything, unless
> it contains a certain string then it will match nothing.
>
> For instance, let's say it should not match "bar", then:
>
> "foo" => "foo"
> "bar " => nil
> "foobar" => nil

some_string !~ /bar/

-philip
B2896675ee61d866e6a5a19407730602?d=identicon&s=25 toulax@gmail.com (Guest)
on 2007-06-28 21:12
(Received via mailing list)
On Jun 28, 4:00 pm, Philip Hallstrom <r...@philip.pjkh.com> wrote:
>
> -philip

Yeah, that's the easy way, is it possible to do that with the regular
expression alone and not use !~? The reason is because this is not
exactly for Ruby and I don't have another option.

Thanks
D0338c0de4cb3c5c17300396159933d1?d=identicon&s=25 Axel Etzold (Guest)
on 2007-06-28 21:50
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I think what you want is what's called negative lookahead.
I don't think it's available in Ruby's Regexps anyway,
and resorting to Oniguruma isn't an option as you don't
want to use Ruby anyway. Maybe you just break it down into
more steps as here:

http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/...

Or you look for negative lookahead here:

http://www.regular-expressions.info/lookaround.html

(provided the language you'll eventually use sports this feature
at all).

Best regards,

Axel
B84c2636d7a795ffffe069b75a9c57d5?d=identicon&s=25 Jesse Hk (jessehk)
on 2007-06-28 23:34
Am I missing something here? Why not just test for equality?


irb(main):001:0> "foo" == "foo"
=> true
irb(main):002:0> "foo" == "bar"
=> false
1fba4539b6cafe2e60a2916fa184fc2f?d=identicon&s=25 unknown (Guest)
on 2007-06-28 23:41
(Received via mailing list)
Hi --
>
On Fri, 29 Jun 2007, Jesse Hk wrote:

> Am I missing something here? Why not just test for equality?
>
>
> irb(main):001:0> "foo" == "foo"
> => true
> irb(main):002:0> "foo" == "bar"
> => false

That doesn't cover the case where the string includes the substring
"bar" but also includes other characters.


David
Bc8e3e2f7acb3a7d93c10e235dc0b4e1?d=identicon&s=25 yermej@gmail.com (Guest)
on 2007-06-28 23:46
(Received via mailing list)
On Jun 28, 2:49 pm, "Axel Etzold" <AEtz...@gmx.de> wrote:
> http://www.regular-expressions.info/lookaround.html
>
> (provided the language you'll eventually use sports this feature
> at all).
>
> Best regards,
>
> Axel

Positive and negative lookahead work in Ruby Regexp. I guess that
doesn't help the original poster, but FYI and all.

Jeremy
Aaa2b1f12b65d33422e8cdc48d70c0f9?d=identicon&s=25 Stephen Ball (Guest)
on 2007-06-29 04:25
(Received via mailing list)
On 6/28/07, dblack@wobblini.net <dblack@wobblini.net> wrote:
> > => false
>
> That doesn't cover the case where the string includes the substring
> "bar" but also includes other characters.
>
>
> David
>

In that case it still works, so we have:

"foo" => "foo"

irb(main):001:0> "foo" == "foo"
=> true

"bar " => nil

irb(main):002:0> "foo" == "bar"
=> false

"foobar" => nil

irb(main):003:0> "foo" == "foobar"
=> false

-- Stephen
990bf71a4e84e1145a3131f35656dc18?d=identicon&s=25 List Rb (listx300108u79872)
on 2007-06-29 04:41
(Received via mailing list)
some_string unless some_string.match("bar")
B2896675ee61d866e6a5a19407730602?d=identicon&s=25 toulax@gmail.com (Guest)
on 2007-06-29 05:31
(Received via mailing list)
Like I said it has to be a regular expression, nothing else, any
ideas?
Bc8e3e2f7acb3a7d93c10e235dc0b4e1?d=identicon&s=25 yermej@gmail.com (Guest)
on 2007-06-29 06:15
(Received via mailing list)
On Jun 28, 10:27 pm, "tou...@gmail.com" <tou...@gmail.com> wrote:
>
> > > David
> > irb(main):002:0> "foo" == "bar"
> > => false
>
> > "foobar" => nil
>
> > irb(main):003:0> "foo" == "foobar"
> > => false
>
> > -- Stephen

As Axel said, you need to use a regex with negative lookahead. This
should work:
/b(?!ar)/
as long as whichever language/system you're using supports it. Maybe
there's a better group or forum for your question if you're not using
Ruby.

I would have thought this would work:
/.*(?!bar)/
but it doesn't in Ruby. I would take that to mean 0 or more of any
character not followed by bar, but I guess that's not the case. I
(obviously) don't understand negative lookahead all that well. Can
anybody explain?

Jeremy
B2896675ee61d866e6a5a19407730602?d=identicon&s=25 toulax@gmail.com (Guest)
on 2007-06-29 06:30
(Received via mailing list)
On Jun 29, 1:11 am, "yer...@gmail.com" <yer...@gmail.com> wrote:
>
>
> > > => true
>
> /.*(?!bar)/
> but it doesn't in Ruby. I would take that to mean 0 or more of any
> character not followed by bar, but I guess that's not the case. I
> (obviously) don't understand negative lookahead all that well. Can
> anybody explain?
>
> Jeremy

Thanks Jeremy, as you said this doesn't seem to work, but thanks for
pointing me to the right track. Also if anyone can suggest a group
more specific to regexp I'm all ears.
Bc8e3e2f7acb3a7d93c10e235dc0b4e1?d=identicon&s=25 yermej@gmail.com (Guest)
on 2007-06-29 06:32
(Received via mailing list)
On Jun 28, 11:11 pm, "yer...@gmail.com" <yer...@gmail.com> wrote:
>
>
> > > => true
>
> /.*(?!bar)/
> but it doesn't in Ruby. I would take that to mean 0 or more of any
> character not followed by bar, but I guess that's not the case. I
> (obviously) don't understand negative lookahead all that well. Can
> anybody explain?
>
> Jeremy

Sorry, but I was wrong. I /b(?!ar)/ isn't right because it would only
match a string if it contains a 'b'. I would have thought /.*(?!bar)/,
but as I said, that doesn't work in Ruby and probably shouldn't now
that I've thought about it more.

Probably just ignore everything I've said except for the fact that
you'll need negative lookahead. I think. Oh, and the part about asking
in a more appropriate group if you aren't really looking for a Ruby
regex.

Jeremy
E0d864d9677f3c1482a20152b7cac0e2?d=identicon&s=25 Robert Klemme (Guest)
on 2007-06-29 11:30
(Received via mailing list)
On 29.06.2007 05:27, toulax@gmail.com wrote:
> Like I said it has to be a regular expression, nothing else, any
> ideas?

Please disclose more detail on the environment you are using and what
exactly you want to do.  Otherwise it's difficult to come up with
suggestions.  If it is a different programming language it might also be
a good idea to post to a specific forum for that language.

Kind regards

  robert
1fba4539b6cafe2e60a2916fa184fc2f?d=identicon&s=25 unknown (Guest)
on 2007-06-29 12:56
(Received via mailing list)
Hi --

On Fri, 29 Jun 2007, Stephen Ball wrote:

>> > irb(main):002:0> "foo" == "bar"
>
> "foobar" => nil
>
> irb(main):003:0> "foo" == "foobar"
> => false

True; I got it backwards.  What it really doesn't cover is the case of
a string that doesn't contain "bar" but isn't equal to "foo":

"foo" == "abcdef"    # false, but should be true because "abcdef"
                      # does not include "bar"


David
D0338c0de4cb3c5c17300396159933d1?d=identicon&s=25 Axel Etzold (Guest)
on 2007-06-29 16:13
(Received via mailing list)
> Thanks Jeremy, as you said this doesn't seem to work, but thanks for
> pointing me to the right track. Also if anyone can suggest a group
> more specific to regexp I'm all ears.
>
One additional source for Regexp in general and in depth is the book by
Jeffrey Friedl,"Mastering Regular Expressions", published at O'Reilly.
I have to admit that I couldn't solve the problem you were mentioning
in some reasonable time -- and didn't really get a concise clue from
cross-reading that book either - even though it's considered the
encyclopedia
of the subject.
I don't really understand why you can't do the search in two steps:

1.  search for everything that contains bar as a substring and
2 . eliminate it from all results ...?

Best regards,

Axel
3d5d8d4d365f1c40929ee45ac6d88eb7?d=identicon&s=25 Verno Miller (verno)
on 2007-06-30 18:03
> toulax@gmail.com wrote:
> How can I make a regular expression that will match everything, unless
> it contains a certain string then it will match nothing.
>
> For instance, let's say it should not match "bar", then:
>
> "foo" => "foo"
> "bar " => nil
> "foobar" => nil
>
> Thanks in advance


How about a negative look-ahead like the one used here
http://www.ruby-forum.com/topic/37609 ?

If the string contains "bar",  the regex matches the rest of the string
that does contain no further "bar" substring, i.e. /ar.*$/.

ruby -e 'r = /(?!.*bar).*/; puts r.match("foobarxxxfoobarxxx"); puts
r.match("foorab"); puts r.match("yyybarbar")'

=> arxxx
=> foorab
=> ar

Cheers

verno
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