Regex Tester

Test, debug, and understand regular expressions with instant match highlights, live groups, replace preview, split output, match navigation, and browser-only processing. Great for JavaScript patterns, validation rules, find/replace flows, logs, and text parsing.

Professional Regex Lab

Write a regex pattern, toggle flags, paste text, and instantly inspect matches, indexes, capture groups, replacements, and split segments. Built to feel fast, visual, and easy to debug on both mobile and desktop.

🧪 Live Tester 🎯 Match Highlight 🧩 Capture Groups 🔁 Replace Preview ✂️ Split Output 🔒 Browser Only

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Upload logs, plain text, CSV, JSON, code, or notes and test your regex instantly against real content.

📄 text / log / code files · ⚡ instant test · 🎯 highlight matches · 🔒 no upload

StatusWaiting

Flagsg

Matches0

Groups0

Input Size0 chars

Primary Output0 chars

Text Workspace & Primary Result

Paste your source text on the left. The right panel shows highlighted matches, replace preview, split results, or raw summary based on your selected primary view.

Pattern idle View idle

Test Text

0 chars 0 lines

Waiting for test text

No file loaded

Primary Output

Highlighted text 0 chars

No result yet

No active match

Regex Snapshot

Quick stats for your current pattern, matches, groups, text size, and replacement output.

Snapshot idle

Regex—

Pattern Length0

Match Count0

Unique Matches0

Replacement Size0 chars

Split Parts0

Errors & Parser Notes

Invalid syntax, unsupported flags, zero-length behavior, and quick debugging details appear here.

Run the regex to see validation notes here.

Best practice

Use g for finding all matches, i for case-insensitive checks, m for line-based text, and capture groups like (...) when you need structured replacement output.

Zero-length patterns can match between characters and create a lot of results. This tool safely limits those cases so your page does not freeze.

Matches & Groups

Inspect every match with its index, matched value, and captured groups.

Matches idle

Run your regex to inspect matches here.

Replace Preview

See the output produced by text.replace(regex, replacement).

Replace idle

Run the regex to see replacement preview here.

Split Preview

See how your text is split when the regex is used with text.split(regex).

Split idle

Run the regex to see split output here.

Keyboard shortcuts

Ctrl + Enter run regex
Ctrl + → next match
Ctrl + ← previous match
Ctrl + L clear all

JavaScript pattern matching with visible flags and replacements

Test a regular expression against representative text while watching flags, boundaries, groups, and worst-case performance

A regex can search, validate, extract, or replace text, but its behavior depends on the JavaScript engine, flags, Unicode mode, and input. Enter the pattern and sample, select flags, match or replace mode, view, and presets, then inspect every match, captured group, zero-length result, replacement, and performance risk.

Pattern and flags

Global, case, multiline, dotAll, Unicode, sticky, and indices flags change the search model

The g flag finds repeated matches, m changes anchors around line boundaries, s lets dot include line terminators, u improves Unicode interpretation, y requires a match at the current position, and d exposes indices. Enable only flags the production code uses and copy the final pattern with them.

Boundaries and groups

Anchors, word boundaries, lookarounds, and captures need realistic edge cases

A match in one sample does not prove validation. Test empty text, extra prefixes or suffixes, line endings, accented letters, emoji, repeated delimiters, and invalid near-matches. Capturing parentheses affect replacement references; use noncapturing groups when grouping alone is intended.

Performance

Nested ambiguity and backtracking can turn an accepted pattern into a denial-of-service risk

Long untrusted input can expose catastrophic backtracking in patterns with overlapping repetitions. Prefer bounded or unambiguous structures, limit input length, and benchmark worst cases in the actual engine. Use dedicated parsers for URLs, HTML, JSON, or programming languages when their grammar already has a proven parser.

Practical review

Regex acceptance check

Test positive, negative, boundary, Unicode, and long-input cases before integration.

Copy and review the exact pattern plus flags used by production.
Test empty values, near misses, multiline input, Unicode, and repeated delimiters.
Inspect captured groups, zero-length matches, and replacement references.
Benchmark long adversarial input and avoid ambiguous nested quantifiers.
Use a structured parser when the target format is not safely handled by regex.

Technical references: MDN JavaScript regular expressions guide

Regular expression questions

Flags, anchors, groups, zero-length matches, Unicode, replacement, and performance

What does the global regex flag do?

It allows repeated matching through the input. In stateful JavaScript RegExp use, lastIndex behavior can matter across calls, especially with global or sticky patterns.

What is the difference between ^ and $ with multiline mode?

Without m they relate to the whole input boundaries; with m they can also match around line terminators. Test the exact newline forms in your data.

Why does my regex match an empty string?

An optional or zero-or-more path may allow no characters. Global replacement with such a pattern can insert content at many positions, so inspect each match.

How do capture groups affect replacement?

Captured substrings can be referenced in replacement output using engine-specific syntax. Parentheses used only for precedence can be made noncapturing to keep group numbering stable.

Does JavaScript regex handle emoji and Unicode letters automatically?

Behavior depends on Unicode mode, escapes, properties, and how the pattern treats code units or code points. Use the u flag where appropriate and test real scripts.

What is catastrophic backtracking?

It is excessive retry work caused by ambiguous pattern paths on certain inputs. It can freeze a UI or create a denial-of-service risk; simplify and benchmark the pattern.