Free Regex Tester — Test Regular Expressions in Your Browser

Regular expressions are powerful — and notoriously hard to get right on the first try. SimpleTools Regex Tester lets you write a regex, paste test text, and instantly see every match highlighted and every capture group extracted. No page reloads, no server calls.

Why Test Regex in a Dedicated Tool?

Writing regex directly in code has a slow feedback loop: write the pattern, run the code, check output, adjust the pattern. A dedicated regex tester collapses this loop to milliseconds — you see matches as you type each character of your pattern.

Beyond speed, visual feedback makes regex debugging much more intuitive:

See exactly what text matched (not just a true/false result)
Inspect which capture groups captured which content
Test edge cases by adding more test text without touching code
Try the replacement pattern before using it in code

Features

Live matching: Matches highlight as you type the regex
Global and single match modes: Toggle g flag to find all matches or just the first
Flag controls: Toggle i (case-insensitive), m (multiline), s (dotall) individually
Capture group display: See each numbered and named group with its matched value
Match list: See all matches listed with their start position and length
Replacement preview: Enter a replacement string and see the result of replace()/replaceAll()
Named groups: Support for (?<name>...) syntax
Lookahead/lookbehind: Full JavaScript regex feature support

Common Regex Flags Explained

Flag	Meaning	When to Use
`g`	Global — find all matches	Almost always for multiple occurrences
`i`	Case-insensitive	Email matching, search, usernames
`m`	Multiline — `^` and `$` match line start/end	Processing line-by-line text
`s`	Dotall — `.` matches newlines too	Matching across multiple lines

How Regex Testing Works

The tool uses JavaScript’s built-in RegExp object:

const regex = new RegExp(pattern, flags);
const matches = [...text.matchAll(regex)];

Match positions are used to highlight text with HTML <mark> elements for the match, and distinct highlights for each capture group. All of this is pure DOM manipulation — no server, no external service.

Privacy Advantage

Your test text might contain real data — log lines with usernames, emails, IP addresses, or structured records from production systems. When you test regex against real data in online tools that send your text to their servers, that data is transmitted over the internet.

SimpleTools Regex Tester:

✅ Your regex patterns and test text stay in your browser
✅ Works fully offline
✅ No account, no logging
✅ Completely free

How to Use the Regex Tester

Go to simpletools.one/regex-tester
Enter your regex pattern in the pattern field (without delimiters)
Select the flags you need (g, i, m, s)
Paste your test text in the large text area
Matches are highlighted immediately
The match list shows each match with its position and capture groups
To test replacement: enter your replacement string and see the transformed output

Regex Cheat Sheet

Pattern	Matches
`\d+`	One or more digits
`\w+`	One or more word characters (a-z, 0-9, _)
`[A-Z][a-z]+`	Capitalised word
`\b\w+\b`	Complete words
`(?<=@)\w+\.\w+`	Domain in email (lookbehind)
`^.+$`	Full lines (with m flag)
`(\d{4})-(\d{2})-(\d{2})`	ISO date with capture groups
`(?<year>\d{4})-(?<month>\d{2})-(?<day>\d{2})`	ISO date with named groups

Common Use Cases

Validate input formats: Email addresses, phone numbers, postal codes, URLs
Parse log files: Extract timestamps, error codes, IP addresses from log lines
Find and replace in code: Transform variable naming conventions, update API endpoints
Data cleaning: Identify and remove unwanted characters, extra whitespace, or malformed records
Text extraction: Pull specific values from unstructured text

Test your regular expressions at simpletools.one/regex-tester — instant visual feedback, completely private, and free.