# Free Regex Tester (JavaScript) Online | Alher Tech

> Free online JavaScript regex tester. Test patterns, see matches and capture groups highlighted in real time. Supports flags and multi-line mode.

- Canonical page: https://alhertech.com/en/regex-tester/
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- Contact: https://alhertech.com/en/contact/

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Test JavaScript regular expressions live. Highlights matches, captures, and shows errors as you type.

Free online JavaScript regex tester. Test patterns, see matches and capture groups highlighted in real time. Supports flags and multi-line mode.

## Frequently asked questions

### Is this regex flavor the same as Python / PCRE?

No. This tester uses JavaScript's RegExp, which is close to but not identical to PCRE. Notable differences, no lookbehind until ES2018, no atomic groups, and different Unicode property support. Test with the engine you'll actually deploy on.

### What do the flags g, i, m, s, u, y mean?

g = global (all matches), i = case-insensitive, m = multi-line (^ and $ match each line), s = dotAll (. matches newlines), u = Unicode mode (correct handling of code points), y = sticky (match at lastIndex only).

### Why is my regex catastrophically slow?

Nested quantifiers on overlapping alternatives cause exponential backtracking (e.g. (a+)+b on 'aaaaaaaa!'). Restructure to use atomic-like patterns, anchors, or possessive qualifiers, or switch to a non-backtracking engine like RE2.

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