Regex Tester

Our Regex Tester lets you write a regular expression and see, instantly, everywhere it matches inside a block of text. Matches are highlighted inline as you type, and every match is broken out below with its position and any capture groups — no install, no account, nothing sent to a server.

How to Use the Regex Tester

1. Write Your Pattern

• Type a regular expression into the pattern field — no need to wrap it in slashes
• Toggle the flags you need: g (global), i (ignore case), m (multiline), s (dotall)
• Invalid patterns show the exact JavaScript error instead of failing silently

2. Paste Your Test String

• Drop in any block of text — log lines, sample data, a whole document
• Matches are highlighted inline the moment you stop typing
• Nothing about the pattern or the text ever leaves your browser

3. Read the Match List

• Each match is listed with its character index and the exact matched substring
• Capture groups (parenthesized parts of the pattern) are listed underneath each match
• A running match count tells you at a glance how many hits the pattern found

Key Features

Live Highlighting

• Inline <mark>-style highlighting inside the rendered test string
• Updates as you edit the pattern, the flags, or the text
• Highlights every match, not just the first, when g is enabled

Flag Support

• g — global, find every match instead of stopping at the first
• i — case-insensitive matching
• m — ^ and $ match the start/end of each line, not just the whole string
• s — . also matches newline characters

Capture Groups

• Numbered groups from (...) are extracted and shown per match
• Groups that didn't participate in a match are labeled clearly instead of showing blank

Error Handling

• Malformed patterns (unbalanced parentheses, invalid character classes, bad quantifiers) are caught
• The browser's own RegExp error message is shown, so you know exactly what to fix

Use Cases

1. Web Development

• Validating form input patterns before wiring them into production code
• Debugging why a regex isn't matching the way you expect
• Prototyping extraction patterns for log parsing or scraping

2. Data Cleaning

• Finding inconsistent formatting across a dataset
• Testing a pattern against edge cases before running it over a large file
• Confirming capture groups line up with the fields you actually want

3. Learning Regex

• Seeing immediately how a flag changes matching behavior
• Building up a complex pattern piece by piece with instant feedback
• Understanding why a pattern over- or under-matches

Technical Features

• Native Engine: Uses the browser's built-in JavaScript RegExp, not a reimplementation, so behavior matches what you'll get in real JS code
• Real-Time Processing: Highlighting and the match list update on every keystroke
• Safe Zero-Length Handling: Patterns that can match an empty string (like a*) don't hang the page
• Error Handling: Invalid syntax is reported clearly instead of throwing an uncaught exception

Why Use Our Regex Tester

1. No Server Round-Trip

• Matching happens instantly in your browser — no network latency, no rate limits
• Works the same offline as it does online

2. Accurate to JavaScript

• If you're writing a regex for JS code (browser or Node), this tests the exact engine you'll ship with
• Some features differ slightly from PCRE, Python's re, or other flavors — this tool reflects JS behavior specifically

3. Privacy Focused

• Client-side processing only
• No data storage, no logging, no analytics on what you type into the pattern or text fields

Understanding Regular Expressions

What Is a Regular Expression?

A regular expression (regex) is a compact pattern language for describing sets of strings — used for validating input, searching text, and extracting structured pieces out of unstructured text.

Common Regex Building Blocks

1. Character classes: [a-z], \d, \w match a category of characters
2. Quantifiers: *, +, ?, {n,m} control how many times something repeats
3. Anchors: ^ and $ pin a match to the start or end of a string (or line, with m)
4. Groups: (...) capture a sub-match; (?:...) groups without capturing

Best Practices

1. Start narrow, then widen: A pattern that's too permissive will match things you didn't intend
2. Test edge cases: Empty strings, extra whitespace, and unexpected casing catch most regex bugs
3. Use groups deliberately: Only capture what you actually need to extract
4. Watch for catastrophic backtracking: Nested quantifiers on overlapping patterns can be slow on pathological input

Benefits

1. Precision: A well-built regex validates or extracts exactly the shape of data you care about
2. Portability: The same pattern language (with minor flavor differences) works across most programming languages
3. Speed: Once compiled, regex matching is fast even for large chunks of text

Remember: a regex that matches your test cases isn't automatically correct for every input — always test against a few unusual or malformed examples, not just the happy path.