1. 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.

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

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

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

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

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

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

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

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

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

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

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

  13. lookahead assertions in js regular expressions