JavaScript¶

// ── test — returns boolean ──────────────────────────────────
/^\d+$/.test("123")               // true
/^\d+$/.test("abc")               // false

// ── exec — returns match object (advances lastIndex with /g) ──
const re = /\d+/g;
let m;
while ((m = re.exec("12 and 34")) !== null) {
  console.log(m[0], m.index);    // "12" 0, "34" 7
}

// ── match — all matches with /g, first match object without ──
"12 and 34".match(/\d+/g)        // ["12", "34"]
"12 and 34".match(/(\d+)/)       // ["12", "12", index:0, groups:undefined]

// ── matchAll (ES2020) — iterator of all match objects ────────
const matches = [..."12 and 34".matchAll(/(\d+)/g)];
// Each element is a full match object with groups

// ── replace — first match (or all with /g) ──────────────────
"hello world".replace(/\w+/g, s => s.toUpperCase());  // "HELLO WORLD"

// ── replaceAll (ES2021) ──────────────────────────────────────
"a-b-c".replaceAll("-", "_");     // "a_b_c"  (string literal only)

// ── split ───────────────────────────────────────────────────
"one1two2three".split(/\d/)       // ["one", "two", "three"]
"one1two2".split(/(\d)/)          // ["one","1","two","2",""] — captured delimiters included

// /g flag with exec mutates lastIndex — reuse without resetting is a bug
const re = /\d+/g;
re.exec("12 34");   // ["12"]  — lastIndex = 2
re.exec("12 34");   // ["34"]  — lastIndex = 5
re.exec("12 34");   // null   — resets lastIndex to 0
re.exec("12 34");   // ["12"] — starts over!

// Fix: always reset or use matchAll
re.lastIndex = 0;
// Or: use a fresh regex for one-shot operations

const { groups } = "2024-07-04".match(
  /(?<year>\d{4})-(?<month>\d{2})-(?<day>\d{2})/
);
// groups.year = "2024", groups.month = "07", groups.day = "04"

// Named groups in replacement string
"2024-07-04".replace(
  /(?<y>\d{4})-(?<m>\d{2})-(?<d>\d{2})/,
  '$<d>/$<m>/$<y>'
);  // "04/07/2024"

// /y only matches at re.lastIndex — no scanning
const re = /\d+/y;
re.lastIndex = 3;
re.exec("abc123")   // ["123"] — matched at position 3
re.lastIndex = 0;
re.exec("abc123")   // null    — no digits at position 0

const m = "2024-07-04".match(/(?<year>\d{4})-(?<month>\d{2})/d);
m.indices[0]          // [0, 7]  — full match start/end
m.indices.groups.year // [0, 4]  — named group start/end

// new RegExp takes a string — backslashes must be doubled
const pattern = "\\d{4}";          // string contains: \d{4}
const re = new RegExp(pattern, "g"); // regex: /\d{4}/g

// Escaping user input for use in dynamic regex
function escapeRegex(str) {
  return str.replace(/[.*+?^${}()|[\]\\]/g, '\\$&');
}
const userSearch = "price (USD)";
const re = new RegExp(escapeRegex(userSearch), "gi");