Lookbehind in JS regular expressions

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.

urlpattern api: hasregexpgroups

javascript built-in: regexp

javascript built-in: regexp: [symbol.match]

javascript built-in: regexp: [symbol.matchall]

javascript built-in: regexp: [symbol.replace]

javascript built-in: regexp: [symbol.search]

javascript built-in: regexp: [symbol.species]

javascript built-in: regexp: [symbol.split]

javascript built-in: regexp: compile

javascript built-in: regexp: compile: throws `typeerror` when used in `regexp` subclass or with cross-realm mismatch.

javascript built-in: regexp: dotall

javascript built-in: regexp: escape

javascript built-in: regexp: exec

javascript built-in: regexp: flags

javascript built-in: regexp: global

javascript built-in: regexp: global: prototype accessor property (es2015)

javascript built-in: regexp: hasindices

javascript built-in: regexp: ignorecase

javascript built-in: regexp: ignorecase: prototype accessor property (es2015)

javascript built-in: regexp: `regexp.input` (`$_`)

javascript built-in: regexp: lastindex

javascript built-in: regexp: `regexp.lastmatch` (`$&`)

javascript built-in: regexp: `regexp.lastparen` (`$+`)

javascript built-in: regexp: `regexp.leftcontext` (`$``)

javascript built-in: regexp: multiline

javascript built-in: regexp: multiline: prototype accessor property (es2015)

javascript built-in: regexp: `regexp.$1-$9`

javascript built-in: regexp: `regexp()` constructor

javascript built-in: regexp: `regexp.rightcontext` (`$'`)

javascript built-in: regexp: source

javascript built-in: regexp: source: "(?:)" for empty regexps

javascript built-in: regexp: source: line breaks and slashes are escaped

javascript built-in: regexp: source: prototype accessor property (es2015)

javascript built-in: regexp: sticky

javascript built-in: regexp: sticky: anchored sticky flag behavior per es2015

javascript built-in: regexp: sticky: prototype accessor property (es2015)

javascript built-in: regexp: test

javascript built-in: regexp: tostring

javascript built-in: regexp: tostring: line breaks and slashes are escaped

javascript built-in: regexp: unicode

javascript built-in: regexp: unicodesets

lookahead assertions in js regular expressions