RegexValidator should accept a new parameter to perform reversed validation

[Si Feng]

In current implementation, RegexValidator only raises ValidationError when pattern DOES NOT match regex, while it is pretty common to use a "reversed" RegexValidator that raises ValidationError when pattern MATCHES regex. A typical scenario is to catch potential XSS inputs in form field validation (if it matches then the validator should panic).

Technically such reversed match could be performed by tweaking the regex itself, however in real world, not everybody is a regex master and there are a lot of people who may prefer a more straightforward solution as simple as "not some_condition".

In my own projects, I've written a ReversedRegexValidator by subclassing RegexValidator and overriding the call() method to change it's behavior (basically it just copied everything and then removed the "not" statement). It worked well, however there are some problems:

1. RegexValidator uses some Django internal utils that are not documented during upgrades. For example, in Django 1.4.x, RegexValidator used smart_unicode() from utils.encoding, while in Django 1.5.x, it changed to force_text(). The custom ReversedRegexValidator will need to be upgraded as well for such internal change.
2. Overriding the whole call() method in order to just remove (or have) a "not" operation seems to be too much. But that's the current problem with RegexValidator.

So my suggestion is to add a new parameter, say "reverse", to RegexValidator. By default it's False and won't change anything to existing codes, but a user can very easily change it's matching behavior by passing reverse=True.

I've had my changes ready for review. Test cases have been updated as well.

[Si Feng]

​https://github.com/django/django/pull/1387

[Aymeric Augustin]

I'm not strongly against this idea in general, however, I'm very concerned about the rationale.

A blacklist implemented with a regex is a textbook example of the worst possible way to defending against XSS!

[Baptiste Mispelon]

Hi,

I think reverse is a confusing name for this feature because it already has a different meaning for lists (consider the reversed builtin or the reverse argument to sorted). Maybe something like invert would work better?

I also wonder if a separate validator wouldn't be a better approach since the two are fundamentally different. What do you think?

Finally, as noted by claudep, your patch will need documentation too: a new entry in the ref/validators page as well as a mention in the release notes for 1.7.

Thanks

[Si Feng]

Replying to devfeng:

​https://github.com/django/django/pull/1387

Updated, please review. :)

[Tim Graham]

Patch needs updating to merge cleanly.

[Si Feng]

Replying to timo:

Patch needs updating to merge cleanly.

​https://github.com/django/django/pull/2243

[Si Feng]

Replying to devfeng:

Replying to timo:

Patch needs updating to merge cleanly.

​https://github.com/django/django/pull/2243

PR updated per Tim's comments.

[Tim Graham <timograham@…>]

In b102c27ff4d21ea6262e600227530f75337a5df2:

Fixed #20784 -- Added inverse_match parameter to RegexValidator.