JavaScript RegExp toString() Method

What Is toString()?

RegExp.prototype.toString() returns a readable string of the regex in /pattern/flags form. It does not search text — it only describes the RegExp for logs, errors, and debugging.

regex.toString()  → "/pattern/flags"

/hello/gi.toString()     → "/hello/gi"
new RegExp("a+b", "i")   → "/a+b/i"

source  → pattern only ("hello")
flags   → flags only ("gi")
toString → both with slashes ("/hello/gi")

Does not match text. Does not change lastIndex.

How to Use toString()

Call it on any RegExp when you need a string description.

1. Start with a regex literal or new RegExp(…).
2. Call regex.toString() (or coerce with String(regex)).
3. Use the result in logs, error messages, or UI debug panels.
4. For rebuilding a regex, use source + flags instead of parsing the string.

Pseudocode

regex = /pattern/flags

result = "/" + regex.source + "/" + regex.flags
return result

// String(regex) and `${regex}` also call toString()
// No matching. No lastIndex updates.

Cheat Sheet

Keep these facts handy while logging or debugging regexes.

Live Preview

Pick a regex and see toString(), source, and flags side by side.

regex     = /hello/gi
toString() → "/hello/gi"
source     → "hello"
flags      → "gi"
typeof     → "string"

JavaScript Examples

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

const regex = /hello/gi;

console.log(regex.toString());        // "/hello/gi"
console.log(typeof regex.toString()); // "string"

/hello/gi
string

const regex = new RegExp("a+b", "i");

console.log(regex.toString());  // "/a+b/i"
console.log(regex.source);      // "a+b"
console.log(regex.flags);       // "i"

/a+b/i
a+b
i

const zipRule = /^\d{5}$/;

function validateZip(value) {
  if (!zipRule.test(value)) {
    return `Invalid ZIP. Expected pattern ${zipRule.toString()}`;
  }
  return "Valid";
}

console.log(validateZip("90210"));  // Valid
console.log(validateZip("9021"));   // Invalid ZIP. Expected pattern /^\d{5}$/

Valid
Invalid ZIP. Expected pattern /^\d{5}$/

Performance Notes

toString() is cheap and fine for logging. Do not call it in hot match loops — matching itself is the expensive part, and stringifying every time only adds noise.

Key Takeaways