JavaScript RegExp ^ Start Anchor

What Is the ^ Start Anchor?

^ is a zero-width assertion: it matches a position at the start of the string (or each line with the m flag), not a character. Use it for prefixes, line starts, and full-string checks with $.

^pattern     → must start at position 0
^pattern/m   → start of each line (after \n)
^…$          → entire string must match

/^world/     → "world…" ✓   "hello world" ✗
[^abc]       → NOT a start anchor (class negation)

How to Use ^

Put ^ at the front of the pattern so matching can begin only at a start position.

1. Write the text or tokens that must appear at the start (e.g. world, https?, #).
2. Prefix the pattern with ^ — e.g. /^world/.
3. Add m when every line should have its own start (multi-line text).
4. Add $ when nothing may follow after the pattern (full-string validation).

Pseudocode

pattern = ^ <rest>
positions = [0]                    // always: string start

if flag m is set:
    also add index after every \n

for each startPos in positions:
    if <rest> matches from startPos:
        return match

return no match                    // ^ itself consumes nothing

Cheat Sheet

Keep these facts handy while anchoring patterns to the start.

Live Preview

Pick a start-anchored pattern and a sample string to see whether test() passes.

pattern = /^world/
text    = "world peace"
test()  → true

/* ^ pins the match to the first character */

JavaScript Examples

Short, copy-ready snippets. Open DevTools Console, or use Try It Yourself for a live editor.

const phrases = ["world peace", "hello world", "world"];

phrases.forEach((s) => {
  console.log(s, /^world/.test(s));
});

world peace true
hello world false
world true

const text = "start here\nnot start\nstart again";

console.log(/^start/.test(text));    // true (string start)
console.log(/^start/m.test(text));   // true (any line start)
console.log(text.match(/^start/gm)); // ["start", "start"]

true
true
["start", "start"]

const rule = /^\d+$/;

console.log(rule.test("42"));     // true
console.log(rule.test("42abc"));  // false — trailing text
console.log(rule.test("abc42"));  // false — not at start

true
false
false

Performance Notes

Anchors help engines fail fast: if the start does not match, there is nothing to scan. Prefer ^ (and $) over unanchored patterns when you know the match must sit at an edge.

Key Takeaways