1-Byte Developer Tools / Regex

Regex Builder & Tester

Build and test regular expressions with instant feedback, match highlighting and captured groups. The free plan is tuned for everyday strings; premium lifts limits and adds richer diagnostics for heavier logs and trickier patterns.

Runs entirely in your browser · subject text never leaves your device.

Plan: Free Free mode: shorter subjects & simple diagnostics. Premium unlocks longer logs and deeper match detail. Sign in to save common patterns (coming soon)

Pattern & subject Define your regex and the text you want to test it against.

Use the flags below to toggle case sensitivity, multi-line mode and more. We will highlight matches and show captured groups on the right.

Pattern: 0 chars Subject: 0 chars

Regular expression (without the /slashes/)

Flags

i · ignore case g · global matches m · multi-line ^ and $ s · dot matches newlines u · unicode mode

Subject text

We will highlight each match in the subject and list captured groups on the right.

Quick tips

Use ^ and $ to anchor your pattern to the start or end of the line or string.
Wrap parts in brackets ( ) to create capturing groups – we will show them per match.
Prefer explicit quantifiers like {2,5} over endless .* where you can.

Free mode: subject text is best kept under ~8k characters. Premium will raise this so you can work against full log lines and larger files.

Tip: keep patterns small and build up in stages; if it hurts to read, future-you will not thank you.

Matches & breakdown Highlighted matches, basic stats and captured groups.

We stop after a sensible number of matches to prevent runaway patterns. If you see almost-empty matches, check for patterns that can match the empty string.

Ready to run. 0 matches

Subject with matches highlighted

Each match is wrapped in a coloured pill so you can see where the pattern is actually firing.

Paste some subject text and run the regex to see matches highlighted here.

Matches & captured groups

We list each match with its index and any capturing groups. Use this to confirm your pattern is catching exactly what you think it is.

No matches yet. Run the regex to see results here.

Pattern summary

When you run a pattern, we will echo it here with its flags and some basic hints about anchors and potential pitfalls.

Free mode: roughly the first 8k characters of the subject are used, and we cap matches to keep things responsive. Premium raises these limits and surfaces more group detail per match.

Back to tools hub

Cookie preferences