Module aquarium.urlscheme.EscapingAmpersands

How to escape &'s in URLs.

Please read about Ampersands (&'s) in URLs.

The short answer is, if you have a URL with more than one parameter, you should wrap it with $htmlent when you embed it in HTML if you want to pass DTD validation. If you don't care, then it really won't matter. What follows is an explanation of why I can't make it any easier on you.

1. You must escape &'s in URLs in order to pass DTD validation. Per the spec, a browser could look at http://a.com/?a=b©=2 and interpret the ©= as part of value of the a variable instead of a new variable named copy; because © is an HTML entity.

2. To handle #1, Aquarium use to escape the &'s in every URL automatically.

3. However, #2 broke redirects if you redirected to a URL with more than one parameter. I so rarely did this, that I didn't know about the bug for a good year.

4. To fix the bug in #3, we came up with a scheme to always escape &'s but deal with the redirect case specially. Now imagine if you create a HTML link whose URL has a parameter named referrer that is set to your current URL, which itself happens to have two parameters. When the user clicks on the link, Aquarium now has a GET parameter named referrer that is a URL. The programmer can use that URL directly in HTML (in which case the &'s must be escaped) or he might redirect to it (in which case the &'s must not be escaped). The programmer is never going to remember whether the URL is already escaped (per #3, it already is) and whether he needs to escape it for a link or unescape it for a redirect. His brain would core dump.

   When it comes to escaping things, a good general rule is to escape things at the last possible moment. By violating this rule, bad things were happening.

5. We could force engineers to wrap every URL in HTML with htmlent, but that would suck. Too much existing code doesn't.

6. Browsers are smart, and most of the time, if you don't escape the &'s, the browser won't get confused. In fact, you can't generate a URL like http://a.com/?a=b©=2 with Aquarium anyway, because the ; will get urlencoded to %3B. Hence, it's not possible to get Aquarium to generate a URL that would confuse the browser. Programmers who are worried about passing DTD validation and have a URL with more than one parameter will just have to use $htmlent. That's better than forcing every programmer to think about the problem every time he generates a URL since, practically speaking, the warning is pedantic.