How to extract a parameter from a URL using a regex pattern

Question

I’m having trouble with a matching expression. I want to extract the "code" parameter from a link I extracted using Nokogiri, so I tried:

event_id = a.attr("href").match(/\?code=(\d+)/)[1]

Unfortunately what is extracted is the entire query string:

?code=768140119

What is the proper way to just get the value of the parameter and nothing else?

And if you use a.attr("href")[/\?code=(\d+)/, 1]? Or just a.attr("href")[/\d+/] (if there is only 1 sequence of one or more digits)? — Wiktor Stribiżew
– Wiktor Stribiżew, Commented Sep 13, 2016 at 20:47
Can you show the input and the output? Are you really accessing [1] of the match? — Pascal
– Pascal, Commented Sep 13, 2016 at 21:52
It does work? rubular.com/r/ED3hgfEzuF. I also tried it on ruby 2.3.1. — nzajt
– nzajt, Commented Sep 13, 2016 at 22:57
THe only reason "[/\d+/]" doesn't work is there may be multiple sequences of numbers. The only one I care about is the one following "code=". — Dave
– Dave, Commented Sep 14, 2016 at 1:46

the Tin Man · Accepted Answer · 2016-09-14 00:19:14Z

4

Don't use regular expressions, use a well-tested wheel.

Ruby's URI class is your friend, in particular decode_www_form:

require 'uri'

uri = URI.parse('http://foo.com?code=768140119')
uri.query # => "code=768140119"
URI.decode_www_form(uri.query) # => [["code", "768140119"]]
URI.decode_www_form(uri.query).to_h # => {"code"=>"768140119"}

As for extracting the value of a parameter of a tag, Nokogiri makes it easy, just treat the Node like a hash:

require 'nokogiri'

doc = Nokogiri::HTML("
<html>
  <body>
    <a href='path/to/foo'>bar</a>
  </body>
</html>
")

doc.at('a')['href'] # => "path/to/foo"

You don't need to waste time typing attr(...).

edited Sep 14, 2016 at 0:19

answered Sep 14, 2016 at 0:09

the Tin Man

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2 Comments

Dave Over a year ago

THe URL is a relative one (e.g. "../results?code=abcde"). Will this still work with a relative URL?

the Tin Man Over a year ago

What did you try? "How much research effort is expected of Stack Overflow users?" is important.

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