Replacement and extraction with callbacks

Understand the mechanism

matchAll returns an iterator and requires g on its regex argument. Results include captures and indices, unlike the capture-free result array from global match.

String replacements interpret sequences such as $1 and $& specially. A callback’s returned string is used as result text, which is useful for literal user-provided replacement values.

We extract simple key-value pairs and transform date-shaped text from YYYY-MM-DD into DD/MM/YYYY. Reformatting is not calendar validation; a date such as 2026-02-31 still requires a semantic check.

Runnable example

const pairs = [..."a=12 b=7".matchAll(/([a-z])=(\d+)/g)]
  .map(m => [m[1], Number(m[2])]);
console.log(JSON.stringify(pairs));
console.log("2026-09-29".replace(/^(\d{4})-(\d{2})-(\d{2})$/, (_, y, m, d) => d + "/" + m + "/" + y));
const literalReplacement = "$&";
console.log("a.b".replace(/\./g, () => literalReplacement));
console.log("a.b".replace(/\./g, literalReplacement));

Trace and verify

Extraction yields a with 12 and b with 7. The date becomes 29/09/2026. The callback produces a$&b, whereas a string replacement interprets $& as the matched dot and leaves a.b.

Common mistakes and boundaries

Do not interpolate unchecked search text into a regex. For literal text replacement, string-form replaceAll with a callback avoids constructing a regex at all.

Practice challenge

Replace every dot with the literal text $1 and predict the output.

Reference solution and reasoning

Use "a.b.c".replaceAll(".", () => "$1") to produce a$1b$1c, avoiding both regex escaping and replacement-group interpretation.

Further reading

• MDN regular expressions