SyntaxError: Invalid regular expression: /x/: Unterminated group
A regex pattern has invalid syntax — unbalanced parentheses/brackets or unescaped special characters.
Exact message you may see
Invalid regular expressionUnterminated groupNothing to repeat
Seen on:
JavaScript
Meaning
Building a RegExp from user input without escaping characters like ( [ * + ? breaks the pattern. Lookbehind isn’t supported in very old browsers.
Common causes
- Unescaped special characters from user input
- Unbalanced () or []
- Lookbehind in old Safari versions
⚡ Quick fix
- Escape input: s.replace(/[.*+?^${}()|[\]\\]/g, '\\$&')
- Test patterns in a regex tester
- Avoid lookbehind if supporting old Safari
Detailed fix by platform
JavaScript
- javascript
const safe = term.replace(/[.*+?^${}()|[\]\\]/g, '\\$&'); const re = new RegExp(safe, 'i');
How to diagnose
- Pattern — Which part is reported?
- Source — Built from user input?
🔧 Still not fixed?
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Last updated 14 Oct 2026