"Invalid Regular Expression": Common Causes and How to Fix Each One

1What error message will I see?

The wording differs by language, but all of them are telling you the same thing: the pattern string you passed in isn't valid regex syntax for that engine.

Notice that every engine names the structural problem (an unterminated group, a missing bracket) rather than pointing at a single bad character. That's because regex parsers read left-to-right and only realize something's wrong once they reach the end of the string still expecting a closing token.

2The four most common causes

3Fixes for each cause

// Matching literal [tag] text
const re = /[tag]/;
// This matches a SINGLE
// character: t, a, or g

const re = /\[tag\]/;
// Matches the literal
// text "[tag]"

// userInput = "3.5 (kg)"
const re = new RegExp(userInput);
// Throws: Invalid regular
// expression

function escapeRegex(s) {
  return s.replace(
    /[.*+?^${}()|[\]\\]/g, '\\$&'
  );
}
const re = new RegExp(escapeRegex(userInput));

That last pattern is worth keeping on hand: any time you're building a RegExp from a variable rather than a literal, escape the twelve special characters first — otherwise any special character in the input (a stray parenthesis, a plus sign in a phone number) can throw at runtime.

4How to find and fix it

Paste your pattern into our Regex Tester — it validates as you type, so you'll see immediately whether the expression compiles, plus a plain-English breakdown of what each token does. That makes it much faster to spot an unescaped character or a missing bracket than reading the raw pattern character by character.