What are the basics of regular expressions in JavaScript?

Question presented to candidate: "If you create a regex with the global flag and call .test() on it three times in a row against the SAME short string, does it always return true? Walk me through what actually happens internally."

What a strong answer should cover:

• 📌 Interview term: a regex literal (/pattern/flags) — matches text against a pattern; common methods are .test() (boolean), .exec() (a full match object or null), and the string methods .match()/.matchAll()/.replace().
• 📌 Interview term: the real, direct answer to the prompt — verified directly: with the global (g) flag, .test()/.exec() are genuinely stateful — each call advances a real, persistent lastIndex property on the regex object itself, continuing the search from there on the NEXT call. Verified directly with 3 sequential real .test() calls against "xx": the answer is genuinely not always true — the third call genuinely returned false, because lastIndex had advanced past the end of the string, at which point it automatically resets to 0.
• 📌 Interview term: capture groups — parentheses (...) create numbered capture groups, retrievable from a match — verified directly with a real date-parsing example; named capture groups ((?<name>...)) are also verified directly, retrievable via the match's own .groups object.
• 📌 Interview term: match() with g vs. matchAll() — verified directly: str.match(/pattern/g) returns only the matched STRINGS, losing capture-group/index info, while matchAll() returns full match objects (with groups and index) for every match, genuinely richer information.
• A precise answer names the real, common practical use of a capture-group-referencing .replace() ($1, $2, named-group syntax) for reformatting matched text — verified directly, reformatting a date string using its own captured pieces.

Clarifying questions expected:

• None — this is a definitional/technical question; directly answering the prompt's own 3-calls scenario (and correctly explaining WHY the third call differs) is the strong signal, since global-flag statefulness is the single most commonly missed regex detail.

Code / implementation expected: Yes — the real, sequential 3-call .test() demonstration against the same short string is the clearest, most convincing proof of the exact statefulness behavior.