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.

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

javascript operator: conditional operator (`c ? t : f`)

javascript operator: nullish coalescing operator (`??`)

javascript operator: nullish coalescing assignment (`x ??= y`)

javascript operator: optional chaining operator (`?.`)

regular_expressions: lookahead assertion: `(?=...)`, `(?!...)`

regular_expressions: lookbehind assertion: `(?<=...)`, `(?<!...)`

regular_expressions: modifier: `(?ims-ims:...)`

regular_expressions: named capture group: `(?<name>...)`

regular_expressions: named capture group: `(?<name>...)`: duplicate names in different disjunction alternatives are allowed

regular_expressions: non-capturing group: `(?:...)`

regular_expressions: quantifier: `*`, `+`, `?`, `{n}`, `{n,}`, `{n,m}`

lookahead assertions in js regular expressions