Regex Tester

Test and debug regular expressions in real-time with match highlighting, capture group details, and common pattern presets.

Enter a regex pattern above to see matches…

What is a Regular Expression?

A regular expression (regex or regexp) is a sequence of characters that defines a search pattern. Regex is a powerful tool used in virtually every programming language — JavaScript, Python, Java, PHP, Ruby, Go, and many others — for pattern matching within strings. Whether you're validating form inputs, parsing log files, extracting data from documents, or performing complex find-and-replace operations, regular expressions make the job dramatically simpler.

How Regex Works

At its core, a regex pattern describes a set of strings. The regex engine takes your pattern and attempts to match it against an input string character by character. Special characters like . (any character), * (zero or more), + (one or more), ? (optional), [] (character classes), () (capture groups), and anchors like ^ and $ give you incredible flexibility to define precisely what you're looking for.

Common Regex Patterns

Understanding Regex Flags

Flags modify how the regex engine interprets your pattern. The g (global) flag finds all matches rather than stopping at the first one. The i (case-insensitive) flag makes matching ignore letter case — so /hello/i matches "Hello", "HELLO", and "hello". The m (multiline) flag changes how ^ and $ work, making them match the start and end of each line rather than the whole string. Combining flags like gim gives you maximum flexibility for complex text processing.

Capture Groups and Backreferences

Parentheses in regex do double duty: they group parts of the pattern together and they capture the matched text for later use. For example, (\d{3})-(\d{4}) matches a phone prefix and line number, capturing each part separately. In replacement operations, you can reference these groups with $1, $2, etc. Named groups like (?<year>\d{4}) add even more clarity. Backreferences like \1 let you match the same text that was captured earlier — useful for finding repeated words or matching HTML tags.

Why Use Our Regex Tester?

Writing regex by hand is notoriously tricky — what looks correct often contains subtle bugs. Our tester gives you instant visual feedback as you type, highlighting every match in your test string. You can see match counts, inspect individual capture groups, and toggle flags to understand exactly how your pattern behaves. The preset library gives you battle-tested starting points for common validation tasks. Best of all, everything runs locally in your browser — your data never leaves your machine.

Greedy vs. Lazy Quantifiers — A Common Regex Pitfall

One of the most frequent regex debugging issues is the difference between greedy and lazy matching. By default, quantifiers like *, +, and {n,} are greedy — they match as much text as possible. For example, <.*> applied to <div>hello</div> will match the entire string instead of just <div>. To fix this, add a ? after the quantifier to make it lazy: <.*?> matches each tag individually. This is one of the most common "my regex works but doesn't do what I expected" bugs. Use our tester's real-time highlighting to instantly see whether your pattern is being too greedy.

How to Debug Regex That Matches Too Much (or Too Little)

When a regex isn't behaving as expected, try these troubleshooting steps: (1) Check if your quantifiers are greedy when they should be lazy, or vice versa. (2) Verify that your anchors ^ and $ are placed correctly — a missing ^ can cause partial matches you didn't intend. (3) Test with the multiline flag m if your input spans multiple lines. (4) Look for unescaped special characters — . matches any character, [. matches a literal dot. Our regex tester's instant visual feedback makes each of these debugging passes take seconds rather than minutes.

Whether you're a beginner learning regex for the first time or a seasoned developer debugging a complex pattern, our regex tester helps you get it right faster. Bookmark this page and make it part of your daily development toolkit.

💡 Pro Tips for Regex Mastery

Frequently Asked Questions

Does this tester support all JavaScript regex features?

Yes — it uses the browser's native RegExp engine, so anything valid in JavaScript works: lookaheads (?=...), lookbehinds (?<=...), named groups (?<name>...), unicode property escapes \p{...}, and all standard quantifiers and anchors.

Why doesn't my regex match unicode or emoji characters?

By default, character classes like \w only match ASCII word characters. To match full unicode, add the u flag — this enables unicode property escapes like \p{Emoji} and makes character classes unicode-aware. Note: this tester exposes g, i, and m flags; you can append u by editing the flag string directly in your own code.

Is my test data sent anywhere when I use this tool?

No. All processing runs entirely in your browser using JavaScript's built-in RegExp engine. Your regex patterns and test strings never leave your computer — no network requests, no server-side processing, nothing. You can test sensitive data like API keys, internal logs, or customer information with complete confidence.

Can I use this tester for non-JavaScript regex flavors (Python, Java, PCRE)?

This tester uses JavaScript's regex engine, which covers the vast majority of common patterns. However, some flavors have unique features (Python's (?P&;name>...) named groups, PCRE's (*SKIP)(*FAIL) verbs). For cross-flavor testing, use this as your rapid prototyping tool, then validate the final pattern in your target language's test harness.

How do I extract specific values from log files or structured text?

Use capture groups — wrap the parts you want to extract in parentheses. For example, to pull timestamps and error messages from Apache logs, use [(\\d{4}-\\d{2}-\\d{2} \\d{2}:\\d{2}:\\d{2})]\\s+(\\w+):\\s+(.+). The tester shows each group separately in the "Capture Groups" panel, so you can verify every extraction before pasting the pattern into your code. Add the g flag to extract ALL matches from a multi-line log file at once.

How do I match a pattern only when it's followed by (or not followed by) something specific?

Use lookaheads and lookbehinds. A positive lookahead (?=...) asserts that what follows matches a pattern — e.g., \\d+(?=\\s*USD) matches numbers followed by "USD". A negative lookahead (?!...) asserts the opposite. Positive lookbehinds (?<=...) and negative lookbehinds (?<!...) do the same for text before your match. Lookarounds don't consume characters — they're zero-width assertions, so they won't appear in your match results. This tester's capture-group panel confirms exactly what gets captured.

Why does my regex match empty strings or produce zero-width matches?

This is typically caused by optional quantifiers — * (zero or more), ? (zero or one), or {0,n} — applied without a required character before or after them. For example, /\\d*/g matches the spaces between characters when no digits exist, producing unwanted zero-width results. The fix: either require at least one character with + instead of *, or add a boundary condition like \\b. In this tester, you'll see these as matches that highlight nothing — a clear signal to tighten your quantifier. Note: in engines like Python's re.findall(), zero-width matches cause infinite-loop-like behavior; in JavaScript, they're benign but clutter results.