Regex Lookbehind in Javascript Alternatives

Question

I'm trying to use the following regex in JS:

(?<=@[A-Z|a-z]+,)\s|(?<=@[A-Z|a-z]+,\s[A-Z|a-z]+)\s(?=\[[A-Z|a-z]+\])

which translates to:

match all spaces which are preceded by :

• @
• followed by any number of characters in the range A-Z or a-z
• followed by a comma

OR

match all spaces which are preceded by:

• @

• followed by any number of characters in the range A-Z or a-z

• followed by a comma
• followed by a space
• followed by any number of characters in the range A-Z or a-z

AND are succeeded by:

• [
• followed by any number of characters in the range A-Z or a-z
• ]

However, JS doesn't support lookbehind. Is there any alternative for supporting the above regex in JS or any npm library I can use instead?

So if we have a sentence like
Hi my name is @John, Doe [Example] and I am happy to be here that should become
Hi my name is @John,Doe[Example] and I am happy to be here.
Also, if we have something like
Hi my name is @John, Smith Doe [Example], that should become
Hi my name is @John,SmithDoe[Example].

Answer 1

I've updated my answer on new input

console.clear();

var inputEl = document.querySelector('#input')
var outputEl = document.querySelector('#output')

function rep (e) {
  var input = e.target.value;
  var reg = /@([a-z]+?\s*?)+,(\s+[a-z]+)+(\s\[[a-z]+\])?/gim



  matches = input.match(reg);
  var output = input;

  if (matches) {
    replaceMap = new Map()
    for (var i = 0; i < matches.length; i++) {
      var m = matches[i]
        .replace(/\[/, '\\[')
        .replace(/\]/, '\\]')
      replaceMap.set(m, matches[i].replace(/\s+/gm, ''))
    }
    for (var [s,r] of replaceMap) {
      output = output.replace(new RegExp(s, 'gm'), r) 
    }
  }

  outputEl.textContent = output
}

inputEl.addEventListener('input', rep)
inputEl.dispatchEvent(new Event('input'))

textarea {
  width: 100%; 
  min-height: 100px;
}

<h3>Input</h3>
<textarea id="input">@Lopez de la Cerda, Antonio Gabriel Hugo David [Author]. I'm the father of @Marquez, Maria</textarea>
<h3>Output (initially empty)</h3>
<p id="output"></p>
<h3>Expected result (on initial input)</h3>
<p>@LopezdelaCerda,AntonioGabrielHugoDavid[Author]. I'm the father of @Marquez,Maria</p>

Backup of old answer content (for historical reasons)

It works at least in Chrome with this regex:

/(?<=@[a-z]+,)\s+(?![a-z]+\s+\[[a-z]+\])|(?<=(@[a-z]+,\s[a-z]+))\s+(?=\[[a-z]+\])/gmi

See: https://regex101.com/r/elTkRe/4

But you can't use it in PCRE because it is not allowed to have quantifiers in lookbehinds. They must be of fixed width. See the errors to the right here: https://regex101.com/r/ZC3XmX/2

Solution without look behinds and look aheads

console.clear();

var reg = /(@[A-Za-z]+,\s[A-Za-z]+)(\s+)(\[[A-Za-z]+\])|(@[A-Z|a-z]+,)(\s+)/gm

var probes = [
  '@gotAMatch,     <<<',
  '@LongerWithMatch,        <<<',
  '@MatchHereAsWell,    <<<',
  '@Yup,         <<<<',
  '@noMatchInThisLine,<<<<<',
  '@match, match    [match]<<<<<<<',
  '@    noMatchInThisLine,    <<<<'
]

for (var i in probes) {
  console.log(probes[i].replace(reg, '$1$3$4'))
}

.as-console-wrapper { max-height: 100% !important; top: 0; }