Match pattern in Ruby with Regexp

Question

Lets say we have the following array of strings (this array is a lot bigger):

[
  'http://www.example.com?id=123456',
  'http://www.example.com?id=234567'
]

As you can see, everything up to the first digit is the same in both strings. Is there a way to easily find what both strings have in common and what is different? So that I get a string like 'http://www.example.com?id=' and and array like ['123456', '234567'].

What should happen for ['http://www.example.com?id=123456', 'http://www.example.com?id=123457']? — BroiSatse
– BroiSatse, Commented Aug 21, 2014 at 12:13
Regexp might not be the ideal tool for this. Building a trie is straightforward, not horribly slow, and solves your problem. — Taemyr
– Taemyr, Commented Aug 21, 2014 at 12:13
It should output (as said in the question) 'example.com?id=', since both strings have it in common. — Severin
– Severin, Commented Aug 21, 2014 at 12:14
@Severin Never programmed in Ruby. stackoverflow.com/questions/9042426/… discussed it, but the answer invokes commands that I am not familiar with so I can't vouch for it's function. Trie's are a common data structure so it's fairly googleable. - Also note that it's a bit of an overkill for your problem. — Taemyr
– Taemyr, Commented Aug 21, 2014 at 12:21

Patrick Oscity · Accepted Answer · 2014-08-21 13:25:55Z

2

Here's a method to find the longest common prefix in an array.

def _lcp(str1, str2)
  end_index = [str1.length, str2.length].min - 1
  end_index.downto(0) do |i|
    return str1[0..i] if str1[0..i] == str2[0..i]
  end
  ''
end

def lcp(strings)
  strings.inject do |acc, str|
    _lcp(acc, str)
  end
end


lcp [
  'http://www.example.com?id=123456',
  'http://www.example.com?id=234567',
  'http://www.example.com?id=987654'
]
#=> "http://www.example.com?id="

lcp [
  'http://www.example.com?id=123456',
  'http://www.example.com?id=123457'
]
#=> "http://www.example.com?id=12345"

edited Aug 21, 2014 at 13:25

answered Aug 21, 2014 at 13:10

Patrick Oscity

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1 Comment

Patrick Oscity Over a year ago

Edit: changed max to min, because the longest prefix can only be as long as the shorter one of the strings.

max · Accepted Answer · 2014-08-21 12:58:43Z

# This is an approach using higher level ruby std-lib components instead of a regex.
# Why re-invent the wheel?
module UriHelper
    require 'uri'
    require 'cgi'

    # Take an array of urls and extract the id parameter.
    # @param urls {Array} an array of urls to parse
    # @returns {Array}
    def UriHelper.get_id_params( urls )
        urls.map do |u| 
            puts u
            uri = URI(u)
            params = CGI::parse(uri.query)  
            params["id"].first # returned
        end
    end
end

require "test/unit"
# This is unit test proving our helper works as intended
class TestUriHelper < Test::Unit::TestCase
  def test_get_id_params
    urls = [
        'http://www.example.com?id=123456',
        'http://www.example.com?id=234567'
    ]
    assert_equal("123456", UriHelper.get_id_params(urls).first )
    assert_equal("234567", UriHelper.get_id_params(urls).last )
  end
end

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