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Does JavaScript support positive and/or negative lookaheads/lookbehinds? Which combinations of them? Or, to be more specific:
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The positive lookbehind ( (? <= ) ) and negative lookbehind ( (? <! ) ) zero-width assertions in JavaScript regular expressions can be used to ensure a pattern is preceded by another pattern.
The syntax is: X(?= Y) , it means "look for X , but match only if followed by Y ". There may be any pattern instead of X and Y . For an integer number followed by € , the regexp will be \d+(?=
Introduction to the JavaScript regex lookbehind In regular expressions, a lookbehind matches an element if there is another specific element before it. A lookbehind has the following syntax: (?<=Y)X. In this syntax, the pattern match X if there is Y before it.
Lookahead assertions are part of JavaScript's original regular expression support and are thus supported in all browsers.
Here in 2020 some browsers do also support lookbehind (lookahead has been supported from the beginning):
Source: https://caniuse.com/#search=Lookbehind
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Javascript has support for only positive and negative lookahead with no support whatsoever for lookbehinds, but you can still mimic the latter in Javascript using callbacks.
There is a nice article about this here, actually although this article uses callbacks to provide some sort of an alternative support for lookbehind, the same principle can be used in other languages that support lookbehinds but not variable expressions in them so it is handy trick.
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