Add basic positive lookahead support

[WesTyler]

Addresses basic implementation for #18 .

Basic cases not previously supported that now pass:

const justA = randexp('^(?=a).$');            // justA === 'a'

const positivePattern = '^hi(?= no one)';
const lookAhead = randexp(positivePattern);   // lookAhead === 'hi no one'
lookAhead.match(positivePattern);             // -->> [ 'hi', index: 0, input: 'hi no one' ]

const negativePattern = '^hi(?! no one)';
const noLookAhead = randexp(negativePattern); // noLookAhead === 'hi'
noLookAhead.match(negativePattern);           // -->> [ 'hi', index: 0, input: 'hi' ]
lookAhead.match(negativePattern);             // -->> null

[codecov-io]

Current coverage is 100% (diff: 100%)

Merging #27 into master will not change coverage

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[fent]

This is what the library used to do a2b2ff5

But this only supports very basic lookaheads. I can't think of a use case where someone would prefer to use a basic lookahead to a regular group.

[WesTyler]

Not sure if this will influence whether you want to support very basic lookaheads or not, but here's some context on why I made the PR for such a simple implementation.

The company XO Group I work for is about to open-source a project we've been using internally since March for generating valid payloads from Joi schema. With this v1 release, we are implementing your (awesome!) utility for supporting Joi's regex schema options. We're hoping to cover as broad a base of support as possible with v1.

That being said, I totally understand if you don't think it's worth the very basic support :)

[fent]

It's good to hear the library is being used. Thanks for the PR but, supporting only a basic version of that feature would have people believe that your library fully supports lookaheads, when the basic version doesn't provide any added benefits compared to a regular group. And they might become frustrated when they find out lookaheads are not fully supported.

[WesTyler]

Sure thing! Understood :)

I'll go ahead and close this for cleanliness. I may circle back around later to take a crack at a more complete support. Thanks for the feedback and the discussion!

[JonathanMontane]

@fent I've looked into how to support lookahead for generation, and short of transforming every part of the regex into DFAs and doing a cartesian product of those DFAs, it's not possible to support them properly.