Regexp Replace Callback
replace/replaceAll with a replacer callback whose parameters are left untyped. The replacer signature is (match, offset, string): the first argument is the matched substring (a string), the middle offset a number, and the last the whole subject string. Untyped parameters take those defaults, so string methods dispatch correctly on `match`.
// The matched substring is a string, not a number.
console.log("hello".replace(/l/g, (m) => m)) // hello
// String methods work on the untyped match parameter.
console.log("hello".replace(/l/g, (m) => m.toUpperCase())) // heLLo
// A constant string replacement.
console.log("hello".replace(/l/g, (m) => "X")) // heXXo
// match + numeric offset.
console.log("hello".replace(/l/g, (m, o) => `${m}@${o}`)) // hel@2l@3o
// All three: match, offset, and whole string.
console.log("a1b2".replace(/[0-9]/g, (m, o, s) => `[${m}/${o}/${s.length}]`))
// a[1/1/4]b[2/3/4]
// replaceAll behaves the same way.
console.log("a.b.c".replaceAll(/\./g, (m) => m + "-")) // a.-b.-cView source on GitHubexamples/regexp/regexp_replace_callback.ts