About Regex Tester

A focused alternative to regex101. Fast, client-side, no login — real-time pattern matching without the noise.

What it does

Regex Tester lets you write and debug regular expressions in real‑time, directly in the browser. Matches highlight as you type, capture groups are color‑coded, and execution time is measured in milliseconds using the native performance.now() API.

Why it exists

regex101 is excellent but dense. RegexPal is minimal but dated. This tool sits in between — focused on the essentials, with a clean interface that doesn't get in the way of the work.

Features

  • —Real-time match highlighting with 100ms debounce
  • —Capture group display with color-coded group spans
  • —Flag toggles: g, i, m, s, u, y
  • —Replace mode with backreference support ($1, $2, $&)
  • —20 common patterns in 4 categories
  • —Quick-reference cheat sheet with click-to-insert
  • —Shareable URLs via query parameters
  • —Execution time display

Technical notes

All regex execution uses the native JavaScript RegExp API — no server, no backend, no data sent anywhere. State is stored only in the URL query string when you use the Share button.

How to use

  1. 1.
    Enter a regex pattern. Type your pattern in the "Regular Expression" field at the top. The tool runs against the JavaScript RegExp engine, so standard JS regex syntax applies.
  2. 2.
    Add your test string. Paste or type the text you want to test against in the test string area. Matches highlight in real-time as you type — no need to hit Enter.
  3. 3.
    Toggle flags as needed. Use the flag buttons to switch between case-sensitive (i), multiline (m), global (g), and other modes. Hover each flag for a quick description.
  4. 4.
    Inspect matches and groups. Scroll down to see every match with its position, full text, and any captured groups. Groups are color-coded — group 1 is blue, group 2 is green, and so on.
  5. 5.
    Switch to Replace mode. Click "Replace" in the top-left toggle to enter a replacement string. Use $1, $2, $& for backreferences. The replaced output updates live.
  6. 6.
    Share your regex. Click the "Share" button to copy a URL with your pattern, flags, and test string encoded as query parameters. Paste it anywhere.

Example use cases

Validate email addresses

Check whether a string is a valid email format before submitting a form. Load the Email Address pattern from the Patterns library or write your own.

^[\w.-]+@[\w.-]+\.\w{2,}$

Extract dates from log files

Paste a block of server logs into the test string, write a date pattern, and instantly see every timestamp highlighted across the full text.

\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}

Reformat strings with Replace mode

Use Replace mode to rewrite date formats (YYYY-MM-DD → MM/DD/YYYY), restructure CSV columns, or swap out placeholder text across a block of content.

(\d{4})-(\d{2})-(\d{2}) → $2/$3/$1

Debug a failing regex

If a pattern isn't matching what you expect, paste both the pattern and a sample of the real input here. The match stats and error display make it easy to pinpoint the problem.

Use the cheat sheet sidebar for syntax reference
Open the tool