JavaScript RegExp compile() Method

What Is compile()?

RegExp.prototype.compile(pattern, flags?) recompiles an existing RegExp in place with a new pattern and optional flags. It returns the same object after mutating it.

regex.compile(pattern, flags?)

mutates the same RegExp object
returns that same object
legacy / non-standard

prefer:
  regex = new RegExp(pattern, flags)

How to Use compile()

Study the legacy call, then replace it with new RegExp in real projects.

1. Start with an existing RegExp object.
2. Call regex.compile(pattern, flags) — flags are optional.
3. The same object now has the new source / flags.
4. For new code, assign regex = new RegExp(pattern, flags) instead.

Pseudocode

regex = existing RegExp

// legacy:
regex.compile(pattern, flags?)
// same object updated in place
// lastIndex usually reset to 0

// modern replace:
regex = new RegExp(pattern, flags)

Cheat Sheet

Keep these facts handy when reading legacy regex code.

Live Preview

Recompile a RegExp (when available) and compare with new RegExp.

regex = /abc/i
regex.compile("cat", "g")
→ /cat/g  (same object)

modern: new RegExp("cat", "g") → /cat/g

JavaScript Examples

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

const regex = /cat/i;

if (typeof regex.compile === "function") {
  regex.compile("dog", "g");
  console.log(regex.source); // "dog"
  console.log(regex.flags);  // "g"
  console.log(regex.test("dog")); // true
} else {
  console.log("compile() not available");
}

dog
g
true

const regex = /abc/;
const ref = regex;

if (typeof regex.compile === "function") {
  const result = regex.compile("xyz");
  console.log(result === regex); // true
  console.log(ref.source);       // "xyz"
} else {
  console.log("compile() not available");
}

true
xyz

let regex = /cat/i;

// Legacy — do not use in new code
if (typeof regex.compile === "function") {
  regex.compile("dog", "g");
}

// Prefer — standard and clear
regex = new RegExp("dog", "g");

console.log(regex.source); // "dog"
console.log(regex.flags);  // "g"
console.log(regex.test("dog")); // true

dog
g
true

Performance Notes

Cost is similar to creating a new RegExp. The real issue is portability and surprising mutation — not microseconds. Prefer new RegExp for clarity and standards compliance.

Key Takeaways