Lookaheads generate content and negative lookaheads have no effect

[blixt]

Lookaheads are only assertions that the following character(s) match, but don't actually consume characters. It seems randexp generates random content for them, however:

^(?=a).$ // can only match a single "a" but generates two characters ("a" and a random one)

Negative lookaheads should remove possible matches ahead, but randexp ignores them:

^(?!a)[a-b]$ // can only match "b" but also generates "a"

[georgePadolsey]

example of this behavior in #17

[fent]

Going to unify #15 and #17 into this issue.

Right now, I don't know how this would be accomplished. Specially with groups within groups and groups that come after lookaheads.

I'll have to think about it some more.

[xgbuils]

I think that behaviour of lookahead should be the same as without using lookahead. For example: I think that randexp('x(?=y)') should produce the same kind of strings that randexp('xy') because I think that tautology rule exposed in #19 is important to be acomplished.

It is true that 'xy'.match(/x(?=y)/) === ['x']. But /x(?=y)/.test('x') is false. For that, I think that randexp('x(?=y)') should return 'xy'. Because then, /x(?=y)/.test(randexp(/x(?=y)/)) is true.

In summary I think that lookahead expressions are not useful to generate random strings. Any string produced by regular expression with lookahead can be produced with regular expression without lookaheads.

[blixt]

@xgbuils This issue is different than what you describe. Look again at the example:

var regex = /^(?=a).$/;
'a'.match(regex); // ["a"]
var example = randexp(regex); // "ax"
regex.test(example); // false

[xgbuils]

But /^(?=a).$/ is very weird regular expression. I cannot understand how this regexp works because lookahead should be used after some pattern. If not, this looks like an attempt to do a lookbehind.

[esperancaJS]

I'm having the same issue with
^(?=[a-hj-npr-zA-HJ-NPR-Z0-9]{17}$)(?=.[0-9])(?=.[a-hj-npr-zA-HJ-NPR-Z])([a-hj-npr-zA-HJ-NPR-Z0-9]+)$

[JonathanMontane]

@fent any chance of having look ahead implemented in the near future? They're extremely practical when trying to do logical AND on regexes from multiple sources... (thanks for the awesome work!)

[fent]

I might take a shot at it eventually, but not anytime soon. :/

[WesTyler]

@PedroEsperanca what are you expecting to get back, and what are you actually getting back?

[esperancaJS]

a VIN number.

Something like:

1MEFM50293A615995

(taken from http://randomvin.com/)

[gruhn]

Sorry for awaking this ancient thread but I found a way to side step the problem. Positive look-ahead can be emulated with regex intersections and negative look-ahead can be emulated with regex complements. I started hacking on a library for computing those. Say you want random email addresses with 10-15 characters. Instead of using positive look-ahead you can combine the two constraints using regex intersection:

import { intersection } from '@gruhn/regex-utils'
import RandExp from 'randexp' 

const emailRegex = /^[\w\-\.]+@([\w-]+\.)+[\w-]{2,}$/
const lengthRegex = /^.{10,15}$/

const randexp = new RandExp(intersection(emailRegex, lengthRegex))

for (let i=0; i<10; i++) {
  console.log(randexp.gen())
}

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IbxpWqJR@i.mI
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e@c.iKY.jhT
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AUxy@bZ.AS.J.GO

If you want random email addresses that are NOT 10-15 characters, then you can use complement instead of negative look-ahead:

import { intersection, complement } from '@gruhn/regex-utils'
import RandExp from 'randexp' 

const emailRegex = /^[\w\-\.]+@([\w-]+\.)+[\w-]{2,}$/
const lengthRegex = complement(/^.{10,15}$/)

const randexp = new RandExp(intersection(emailRegex, lengthRegex))

for (let i=0; i<10; i++) {
  console.log(randexp.gen())
}

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