Forum: Ruby Regexp problem

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Albert S. (Guest)
on 2009-03-10 09:19
I want to extract an "id" parameter from the URL. I use the following
code:

  url = "http://example.com/id=12345"
  id = url[ /(?<=id=)(\d+)/ ]
  puts id

But I get a SyntaxError telling me "undefined (?...) sequence".

Why?
The H. (Guest)
on 2009-03-10 09:43
Albert S. wrote:
> I want to extract an "id" parameter from the URL. I use the following
> code:
>
>   url = "http://example.com/id=12345"
>   id = url[ /(?<=id=)(\d+)/ ]
>   puts id
>
> But I get a SyntaxError telling me "undefined (?...) sequence".
>
> Why?

Because ruby 1.8 does not have look-behind.  1.9 has it, and your code
runs on 1.9.

If you need 1.8 with look-behind, I guess you can compile 1.8 with the
Oniguruma regexp engine (which is used in 1.9).
Ryan D. (Guest)
on 2009-03-10 10:16
(Received via mailing list)
On Mar 10, 2009, at 00:17 , Albert S. wrote:

> I want to extract an "id" parameter from the URL. I use the following
> code:
>
>  url = "http://example.com/id=12345"
>  id = url[ /(?<=id=)(\d+)/ ]
>  puts id
>
> But I get a SyntaxError telling me "undefined (?...) sequence".

(?:...)

not

(?<...)

but really, /(\d+)/ will suffice for your example. see:

see http://www.zenspider.com/Languages/Ruby/QuickRef.html#11
7stud -. (Guest)
on 2009-03-10 11:20
Albert S. wrote:
> I want to extract an "id" parameter from the URL. I use the following
> code:
>
>   url = "http://example.com/id=12345"
>   id = url[ /(?<=id=)(\d+)/ ]
>   puts id
>
> But I get a SyntaxError telling me "undefined (?...) sequence".
>
> Why?

url = "http://example.com/id=12345"

pieces = url.split("/")
id = pieces[-1].split("=")[1]
puts id

--output:--
12345
Robert K. (Guest)
on 2009-03-10 11:27
(Received via mailing list)
2009/3/10 The Higgs bozo <removed_email_address@domain.invalid>:
>> Why?
>
> Because ruby 1.8 does not have look-behind.  1.9 has it, and your code
> runs on 1.9.
>
> If you need 1.8 with look-behind, I guess you can compile 1.8 with the
> Oniguruma regexp engine (which is used in 1.9).

Lookbehind is not needed:

09:35:18 ~$ irb
Ruby version 1.8.7
irb(main):001:0> url = "http://example.com/id=12345"
=> "http://example.com/id=12345"
irb(main):002:0> id = url[/id=(\d+)/, 1]
=> "12345"

Depending on what else should be done with the url a more appropriate
tool might be class URI

irb(main):006:0> require 'uri'
=> true
irb(main):007:0> u2 = URI.parse url
=> #<URI::HTTP:0x7ff186bc URL:http://example.com/id=12345>
irb(main):008:0> u2.path
=> "/id=12345"
irb(main):009:0> u2.path[/id=(\d+)/, 1]
=> "12345"

Kind regards

robert
Albert S. (Guest)
on 2009-03-10 12:14
The Higgs bozo wrote:
>>> Why?
>>
>> Because ruby 1.8 does not have look-behind. 1.9 has it, and your code
>> runs on 1.9.

Thanks for the info, Higgs.

Robert K. wrote:
> Lookbehind is not needed:
[snip]
> irb(main):002:0> id = url[/id=(\d+)/, 1]
> => "12345"

Cool! I once knew of the second parameter. Thanks for reminding me of
it.

I wish I could use RI (too slooouuuw on my system) or Fast-RI (I have
it, but at one point it decided to start throwing exceptions).
Albert S. (Guest)
on 2009-03-10 12:19
Albert S. wrote:
> I wish I could use RI (too slooouuuw on my system) or Fast-RI (I have
> it, but at one point it decided to start throwing exceptions).

Oops. It started *raising* exceptions. Everybody knows that in Ruby you
don't *throw* exceptions ;-)
Tom C. (Guest)
on 2009-03-10 12:59
(Received via mailing list)
Albert S. wrote:
>
I've lately been making excellent use of the  MatchData class, so here's
my solution:

irb(main):012:0> m = /\d+/.match("http://example.com/id=12345")
=> #<MatchData "12345">
irb(main):013:0> m[0]
=> "12345"

In other situations, some of the various class methods can be useful:

irb(main):014:0> m.pre_match
=> "http://example.com/id="
irb(main):015:0>

There's a #post_match method also...and more.

t.

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