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Regex assertions match differently with different spacing

I'm new to RegEx and trying to learn via MDN. I've made it to assertions and I've run into some confusion with the Lookahead assertion x(?=y).

The example the MDN gives is:

/Jack(?=Sprat|Frost)/ matches "Jack" only if it is followed by "Sprat" or "Frost".

However - when I've tested this, it doesn't work in either case, until I add spaces around = and \. Here are some tests I ran (Repl link)

let regex = /Jack(?=Sprat|Frost)/;

let nurseryRhyme1 = 'Jack Frost is real.' 

let nurseryRhyme2 = 'Jack, this Frost is real.' 

let nurseryRhyme3 = 'Jack Sprat is not real.'


console.log(nurseryRhyme1.match(regex)) // null
console.log(nurseryRhyme2.match(regex)) // null
console.log(nurseryRhyme3.match(regex)) // null 

regex = /Jack(?= Sprat|Frost)/;

nurseryRhyme1 = 'Jack Frost is real.' 

nurseryRhyme2 = 'Jack, this Frost is real.' 

nurseryRhyme3 = 'Jack Sprat is not real.'

console.log(nurseryRhyme1.match(regex)) // null
console.log(nurseryRhyme2.match(regex)) // null 
console.log(nurseryRhyme3.match(regex)) // matches, returns ['Jack']

regex = /Jack(?= Sprat | Frost)/;

nurseryRhyme1 = 'Jack Frost is real.' 

nurseryRhyme2 = 'Jack, this Frost is real.' 

nurseryRhyme3 = 'Jack Sprat is not real.'

console.log(nurseryRhyme1.match(regex)) // matches, returns ['Jack']
console.log(nurseryRhyme2.match(regex)) // null 
console.log(nurseryRhyme3.match(regex)) // matches, returns ['Jack']

The documentation doesn't mention anything about spaces being necessary and lays out the pattern as x(?=y) (no spaces).

In general, is spacing a necessary part of a regex assertion or am I doing something wrong? I've tended to avoid regex in the past because it looks like chaos and I'm trying to get a better sense of tricks/tips to making it work.

Any guidance is appreciated - I couldn't find this specific topic while searching, but if there's a post I'd love to read it!

Via Active questions tagged javascript - Stack Overflow https://ift.tt/1DRY7Xq
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