URL を置換するだけです

replace_urls.rb

# -*- coding: utf-8 -*-
class String
  AVAILABLE_CHARS_IN_URI = [
    '\;', '\/', '\?', '\:', '\@', '\&', '\=', '\+', '\$', '\,',
    '\-', '\_', '\.', '\!', '\~', '\*', '\\\'', '\(', '\)',
    '\%', '0-1', 'a-z', 'A-Z' '\#'
  ]
  REGEXP_OF_URL = /https?\:\/\/[#{AVAILABLE_CHARS_IN_URI.join('')}]+/
  def replace_urls &blk
    gsub(REGEXP_OF_URL, &blk)
  end
end

require "test/unit"
class TestReplaceURLs < Test::Unit::TestCase
  def setup
    @text = "abcd efg http://example.com hijk lmn https://example.com/index.html opqr stu vwxyz"
    @replaced = "abcd efg [[http://example.com]] hijk lmn [[https://example.com/index.html]] opqr stu vwxyz"
  end
  def test_replace_urls
    assert_equal(@replaced, @text.replace_urls { |url| "[[#{url}]]"  })
  end
end