# Regex Tester

> Regex tester: test regular expressions against sample text and see live matches, capture groups, and errors to debug patterns fast. Free, private, no signup.

- **Canonical URL:** https://boringtoolskit.com/regex-tester/
- **Category:** Everyday Calculators
- **Evidence level:** authored-methodology
- **Reviewed:** July 26, 2026
- **Last verified:** 2026-10-12
- **Privacy:** Inputs and results stay in this browser unless the page clearly labels an external request.

## How it works

Compile the pattern with the given flags using JavaScript's RegExp constructor, then iterate matches over the test string with exec(), collecting each match and its zero-based start index. A missing global 'g' flag returns only the first match; a match limit guards against runaway matches.

## Worked example

For the pattern \b[A-Z]{2,}\b with flags gi against 'The USA and the UK use SHOUTING in TITLES.', the tool finds 9 matches (every word) and lists each with its position, showing how the case-insensitive flag changes the result.

## Assumptions

- The pattern is evaluated with JavaScript's RegExp engine, matching the syntax your browser supports.
- Flag letters are validated to g, i, m, s, u, and y.
- A match limit of 2,000 prevents catastrophic backtracking from freezing the page.
- When 'Escaped' mode is selected, literal \t and \n are converted to real tabs and newlines.

## Common questions

### Why does my pattern only return one match?

Add the g (global) flag. Without it, the engine returns only the first match in the string.

### Does the tool send my pattern or text anywhere?

No. The regex runs entirely in your browser. Nothing you paste is transmitted or stored.

### Which regex flavor does this use?

JavaScript's native RegExp engine — the same syntax used in browsers and Node. Lookbehinds and named groups depend on your browser's engine version.

### My page froze on a huge pattern.

A built-in 2,000-match limit stops runaway matching. Very complex patterns can still be slow; try simplifying or anchoring the pattern.

## Related tools

- /json-formatter/
- /markdown-table-generator/
- /general-scientific-calculator/

## Sources

- [Calculation methodology](/methodology/)
- [Quality evidence and verification standard](/quality-evidence/)

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