enhancement: negative examples?

[tlubz]

Is there a way to generate counter-examples to a regex pattern? This would be incredibly useful for writing tests for format validators. Think validates_format_of for shoulda-matchers.

Conceptually, it would enumerate all strings that don't match the pattern.

Ideal behavior:

/a+/.counter_examples #=> ["", "b", "c", "d", ...]
/hello/.counter_examples #=> ["", "a", "b", "c", ...]
/.*/.counter_examples #=> [] (or nil)

it seems like this is computationally possible at least. the complement of a regular language is also regular, which means that you can enumerate the counter-examples by "negating" the regular expression's deterministic finite automaton representation, and then enumerating examples from that.

https://math.stackexchange.com/questions/685182/complement-of-a-regular-expression

[tom-lord]

I like the idea, but I suppose this problem should be solved be a separate gem - what do you think?

For example, we could have RegexpNegator.new(regex) #=> negated_regex

Or even, if the problem is feasibly doable, this could be a core ruby method - e.g. Regexp#negate? In which case, this gem could be used as follows:

/hello/.negate.random_example

[tom-lord]

...But what does a negated /hello/ actually look like? Without using irregular aspects of regex (i.e. look-arounds), that is. I suspect this may closely relate to this problem I encountered with the absence operator.

Edit -- See also: https://bugs.ruby-lang.org/issues/5588