Regexp problem


#1

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?


#2

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).


#3

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


#4

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


#5

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).


#6

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


#7

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.

Tom C., MS MA, LMHC - Private practice Psychotherapist
Bellingham, Washington, U.S.A: (360) 920-1226
<< removed_email_address@domain.invalid >> (email)
<< TomCloyd.com >> (website)
<< sleightmind.wordpress.com >> (mental health weblog)

#8

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 :wink: