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comp.lang.ruby

Regexp Matching Not Blank And Not A Specific Word?

Andrew Stewart

12/13/2007 6:16:00 PM

Hi,

Please could somebody show me how to write a regular expression that
matches when the input is not blank and it's not a specific word,
e.g. dog.

For example, I would hope for these results:

'' =~ regexp # nil
'dog' =~ regexp # nil
'cat' =~ regexp # not nil

I have tried various permutations and combinations involving (?!re)
but I just can't find the answer. In fact I'm beginning to doubt
that (?!re) works at all on my system (Ruby 1.8.6)...but on balance
it's probably me rather than my Ruby installation!

Thanks and regards,
Andy Stewart

-------
http://airbladeso...
6 Answers

Sebastian Hungerecker

12/13/2007 6:47:00 PM

0

Andrew Stewart wrote:
> In fact I'm beginning to doubt =A0
> that (?!re) works at all on my system (Ruby 1.8.6)...but on balance =A0
> it's probably me rather than my Ruby installation!

It's not you. The ruby 1.8 regexen don't support negative lookahead. You'll
get that in ruby 1.9 or by installing and using the oniguruma regexp-engine
instead of the default ruby 1.8 one.


HTH,
Sebastian
=2D-=20
Jabber: sepp2k@jabber.org
ICQ: 205544826

Phrogz

12/13/2007 7:03:00 PM

0

On Dec 13, 11:47 am, Sebastian Hungerecker <sep...@googlemail.com>
wrote:
> Andrew Stewart wrote:
> > In fact I'm beginning to doubt
> > that (?!re) works at all on my system (Ruby 1.8.6)...but on balance
> > it's probably me rather than my Ruby installation!
>
> It's not you. The ruby 1.8 regexen don't support negative lookahead. You'll
> get that in ruby 1.9 or by installing and using the oniguruma regexp-engine
> instead of the default ruby 1.8 one.

Not true:
C:\>ruby -v
ruby 1.8.6 (2007-03-13 patchlevel 0) [i386-mswin32]

C:\>irb
irb(main):001:0> s = "hello world"
=> "hello world"
irb(main):002:0> s.scan /[eo]./
=> ["el", "o ", "or"]
irb(main):003:0> s.scan /[eo](?!r)./
=> ["el", "o "]

It just doesn't support negative or positive *lookbehinds*.

Phrogz

12/13/2007 7:07:00 PM

0

On Dec 13, 11:16 am, Andrew Stewart <b...@airbladesoftware.com> wrote:
> Hi,
>
> Please could somebody show me how to write a regular expression that
> matches when the input is not blank and it's not a specific word,
> e.g. dog.
>
> For example, I would hope for these results:
>
> '' =~ regexp # nil
> 'dog' =~ regexp # nil
> 'cat' =~ regexp # not nil


irb(main):006:0> tests = [ "", "dog", "cat", "doggone it" ]
=> ["", "dog", "cat", "doggone it"]

irb(main):007:0> tests.each do |str|
irb(main):008:1* hit = (str =~ /^(?!dog).+$|^dog.+$/)
irb(main):009:1> puts "#{str.inspect} => #{hit.inspect}"
irb(main):010:1> end
"" => nil
"dog" => nil
"cat" => 0
"doggone it" => 0

Judson Lester

12/13/2007 8:07:00 PM

0

[Note: parts of this message were removed to make it a legal post.]

On Dec 13, 2007 11:09 AM, Phrogz <phrogz@mac.com> wrote:

> irb(main):006:0> tests = [ "", "dog", "cat", "doggone it" ]
> => ["", "dog", "cat", "doggone it"]
>
> irb(main):007:0> tests.each do |str|
> irb(main):008:1* hit = (str =~ /^(?!dog).+$|^dog.+$/)
> irb(main):009:1> puts "#{str.inspect} => #{hit.inspect}"
> irb(main):010:1> end
> "" => nil
> "dog" => nil
> "cat" => 0
> "doggone it" => 0


Just to shill for a moment:

I was doing that kind of irb stuff so often for regexps in Ruby, I wrote
this:

http://rubyforge.org/projects/re...
http://regexpbench.ruby...

Hope it's useful.

Judson

--
Your subnet is currently 169.254.0.0/16. You are likely to be eaten by a
grue.

Andrew Stewart

12/14/2007 12:59:00 PM

0


On 13 Dec 2007, at 19:09, Phrogz wrote:

> On Dec 13, 11:16 am, Andrew Stewart <b...@airbladesoftware.com> wrote:
>> Hi,
>>
>> Please could somebody show me how to write a regular expression that
>> matches when the input is not blank and it's not a specific word,
>> e.g. dog.
>>
>> For example, I would hope for these results:
>>
>> '' =~ regexp # nil
>> 'dog' =~ regexp # nil
>> 'cat' =~ regexp # not nil
>
>
> irb(main):006:0> tests = [ "", "dog", "cat", "doggone it" ]
> => ["", "dog", "cat", "doggone it"]
>
> irb(main):007:0> tests.each do |str|
> irb(main):008:1* hit = (str =~ /^(?!dog).+$|^dog.+$/)
> irb(main):009:1> puts "#{str.inspect} => #{hit.inspect}"
> irb(main):010:1> end
> "" => nil
> "dog" => nil
> "cat" => 0
> "doggone it" => 0

That's fantastic, thank you!

When I was trying this yesterday, I think I failed to appreciate that
(?!re) doesn't consume the match. Now I know.

Thanks also for the tip in your previous email about Ruby 1.8 and
lookbehinds.

Regards,
Andy Stewart

-------
http://airbladeso...

Andrew Stewart

12/14/2007 1:00:00 PM

0


On 13 Dec 2007, at 20:06, Judson Lester wrote:
>
> Just to shill for a moment:
>
> I was doing that kind of irb stuff so often for regexps in Ruby, I
> wrote
> this:
>
> http://rubyforge.org/projects/re...
> http://regexpbench.ruby...
>
> Hope it's useful.

That looks very useful. I'm often twiddling regexps (time-
consumingly) so I'll give it a try.

Thanks!

Regards,
Andy Stewart

-------
http://airbladeso...