Regex Tester Online — Free Regular Expression Tester, Debugger & Explainer

How to Regex Tester Online

1. 1

   Enter your regular expression in the pattern field at the top of the left panel. The regex is validated in real-time — errors are shown immediately with clear messages.

2. 2

   Toggle regex flags using the flag buttons below the pattern: g (global), i (case-insensitive), m (multiline), s (dotall), u (unicode), v (unicode sets), d (indices), y (sticky).

3. 3

   Paste or type your test string in the text area below the flags. Matches are highlighted instantly with alternating colors.

4. 4

   View match results in the right panel: each match shows the full text, index, length, numbered capture groups, and named capture groups.

5. 5

   Switch between modes using the bottom bar: Test (match highlighting), Replace (substitution preview), Split (string splitting), Explain (regex breakdown tree), or Generate (code snippets in 9 languages).

6. 6

   Use the Cheat Sheet button for an inline regex syntax reference, or browse the Common Patterns library for 30+ battle-tested regex templates.

7. 7

   Copy results with ⌘⇧C, or share your regex and test string via a compressed URL.

Regex Tester Features

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  Real-time match highlighting: all matches highlighted with alternating colors directly in the test string as you type.

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  Full capture group support: view numbered groups ($1, $2, ...) and named groups (?<name>...) for every match.

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  8 regex flags: toggle g, i, m, s, u, v, d, y with instant visual feedback and tooltip descriptions.

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  Replace mode: test string.replace() and string.replaceAll() with substitution tokens ($1, $&, $', $`, $<name>).

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  Split mode: test string.split(regex) and view the resulting array with element count.

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  Regex explainer: visual tree breakdown of your regex structure — groups, quantifiers, anchors, lookarounds, and character classes explained in plain English.

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  Code generation: generate ready-to-use regex code in JavaScript, TypeScript, Python, Go, Java, C#, PHP, Ruby, and Rust.

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  Match statistics: total matches, unique matches, average match length, coverage percentage, shortest and longest match.

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  Inline regex cheat sheet: searchable reference for all regex syntax — character classes, quantifiers, anchors, groups, lookarounds, Unicode properties, and escapes.

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  Common patterns library: 30+ categorized, battle-tested regex templates for emails, URLs, IPs, dates, passwords, HTML tags, and more.

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  Pattern validation: real-time syntax checking with clear error messages for invalid regex.

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  Catastrophic backtracking protection: patterns that would cause excessive execution time are safely terminated.

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  Shareable URLs: gzip-compresses your pattern, flags, and test string into a URL for team sharing.

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  Works entirely in your browser — no regex patterns or test data are ever sent to any server. Your data stays completely private.

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  Keyboard shortcuts for power users: ⌘↵ to run, ⌘⇧R for replace, ⌘⇧E for explain, ⌘⇧G for generate, ⌘⇧H for cheat sheet.

Frequently Asked Questions

Is this regex tester free?

Yes, DevFlow Regex Tester is completely free with no usage limits. All matching, replacing, explaining, and code generation runs directly in your browser — no account required.

Is my data safe?

All regex processing happens entirely in your browser using JavaScript. No patterns, test strings, or results are ever transmitted to a server or stored anywhere outside your browser. Your data stays completely private on your machine.

What regex flavors are supported?

The tool uses JavaScript's native RegExp engine, which supports ECMAScript 2024 features including named capture groups (?<name>...), Unicode property escapes (\p{Letter}), lookbehind assertions (?<=...), the dotall flag (s), the hasIndices flag (d), and the unicodeSets flag (v). The code generation feature translates patterns to equivalent syntax in Python, Go, Java, C#, PHP, Ruby, and Rust.

What is the difference between the g and y flags?

The g (global) flag finds all matches in the string by searching from each position. The y (sticky) flag only matches at the exact position indicated by the regex's lastIndex property — it does not search ahead. Sticky mode is useful for building tokenizers and parsers where you need to match sequentially from a specific position.

What is the v (unicodeSets) flag?

The v flag (ES2024) enables Unicode Sets mode, which extends character classes with set operations: intersection (&&), subtraction (--), and nested classes. For example, [\p{Letter}&&\p{Script=Greek}] matches only Greek letters. The v flag is mutually exclusive with the u flag.

What are named capture groups?

Named capture groups use the syntax (?<name>...) to assign a name to a capture group instead of just a number. For example, (?<year>\d{4})-(?<month>\d{2})-(?<day>\d{2}) captures year, month, and day as named groups. In JavaScript, access them via match.groups.year. Named groups make regex more readable and maintainable.

What are lookahead and lookbehind assertions?

Lookaround assertions check for a pattern without including it in the match. (?=...) is a positive lookahead (match only if followed by ...), (?!...) is a negative lookahead (match only if NOT followed by ...), (?<=...) is a positive lookbehind (match only if preceded by ...), and (?<!...) is a negative lookbehind (match only if NOT preceded by ...). They are zero-width — they do not consume characters.

How does the regex explainer work?

The explainer parses your regex into a tree structure, identifying each construct — groups, quantifiers, character classes, anchors, alternations, and literals. Each node is described in plain English. For example, \d{2,4} is explained as 'Between 2 and 4 digits'. This helps you understand complex regex written by others or verify your own patterns.

What replacement tokens are supported?

The Replace mode supports standard JavaScript replacement tokens: $1, $2, etc. for numbered capture groups, $<name> for named groups, $& for the full match, $` for the text before the match, $' for the text after the match, and $$ for a literal dollar sign.

What is catastrophic backtracking?

Catastrophic backtracking occurs when a regex engine tries an exponential number of paths through the input due to nested quantifiers or ambiguous alternations. For example, (a+)+ against a long string of 'a's followed by a non-matching character can take millions of steps. This tool detects such patterns and safely terminates execution after 100ms to prevent browser hangs.

Can I generate regex code for Python, Go, Java, or other languages?

Yes. The Generate mode produces idiomatic regex code in JavaScript, TypeScript, Python, Go, Java, C#, PHP, Ruby, and Rust. Each snippet uses the language's standard regex library with appropriate flag translation and syntax escaping.

Does it work offline?

Yes. All regex processing is done using JavaScript's built-in RegExp engine. Once the page has loaded, the Regex Tester works without an internet connection.

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