Rails auto_link and certain query strings

Our local newspaper, the San Francisco Chronicle, is occasionally a source for data stories posted to Swivel, and that's how we discovered that there are a couple of edge cases that Rails's auto_link doesn't handle perfectly.

A typical Chronicle link looks something like this: <a href="http://www.sfgate.com/cgi-bin/article.cgi?f=/c/a/2007/08/24/WIJKRIC6E.DTL">. Nothing too peculiar there. However, when passed through the auto_link function, the / character at the beginning of the value of the query string variable causes the entire value to be dropped, leaving us with lots of links to <a href="http://www.sfgate.com/cgi-bin/article.cgi?f=">.

This bug has been noted by Satya among others, and the monkey-patch solution is fairly straightforward. The first step is to write a functional test:

  test("URL querystring variables can begin with a slash") do
    url = 'www.example.com?url=/rails/cannot/deal/with/this/'
    post(:create, :resource => { :name => "test", :link => url })
    get(:show, :id => assigns(:resource).id)
    assert_select("a[href=http://#{url}]")
  end

Our test fails. The URL is truncated at the =.

ActionView's TextHelper includes a regular expression constant AUTO_LINK_RE which tells Rails how to recognize URLs. We prefer not to tinker with the actual vendor code itself to avoid any upgrading gotchas or other weirdness, so I copied the relevant RegEx to /configuration/initializers/core_ext.rb, made the necessary (one-character!) tweak, and overrode the private TextHelper constant with the new Regex. The changed line is in bold:

ActionView::Helpers::TextHelper::AUTO_LINK_RE = %r{
              (                          # leading text
                <\w+.*?>|                # leading HTML tag, or
                [^=!:'"/]|               # leading punctuation, or
                ^                        # beginning of line
              )
              (
                (?:https?://)|           # protocol spec, or
                (?:www\.)                # www.*
              )
              (
                [-\w]+                   # subdomain or domain
                (?:\.[-\w]+)*            # remaining subdomains or domain
                (?::\d+)?                # port
                (?:/(?:[~\w\+@%=\(\)-]|(?:[,.;:'][^\s$]))*)* # path
                (?:\?[\w\+@%&=.;:/-]+)?     # query string
                (?:\#[\w\-]*)?           # trailing anchor
              )
              ([[:punct:]]|<|$|)       # trailing text
             }x

Our test passes, and all is well.