OnSumoTools

Regex Visualizer Online

Test regex in your browser. Your data stays private.

This regex tester runs entirely in your browser. Your test strings, patterns, and data stay on your device from start to finish. Paste a pattern, add your test string, and see every match highlighted in real time with full group capture breakdown. No server round-trips, no data exposure, no account required.

Valid pattern2 matches, 0.32 ms

Match list

Regex is executed in your browser only. Keep in mind that some patterns can be slow on large text.

What is a regex visualizer and when should you use it?

A regex visualizer lets you write, test, and debug regular expressions online by highlighting matches, showing capture groups, and previewing replacements as you edit.

Regex visualization starts with parsing. The tool reads your regex string, identifies each syntax element (literals, metacharacters, quantifiers, groups, anchors), and builds a parse tree that represents the pattern's structure. For example, the pattern \d{3}-\d{4} becomes a tree with two digit-class nodes, each with a quantifier, separated by a literal hyphen.

How do you test a regex pattern online?

Enter a regex pattern and test string, and see real-time match highlighting, capture groups, and an optional replacement preview — entirely in your browser.

Pattern: (\d{4})-(\d{2})-(\d{2}). Flags: g. Test string: Release date: 2026-05-28 and 2025-12-01. The tool returns two matches. Match 1: 2026-05-28 at position 14, groups year=2026, month=05, day=28. Match 2: 2025-12-01, groups year=2025, month=12, day=01. With replacement string $<day>/$<month>/$<year>, the preview shows: Release date: 28/05/2026 and 01/12/2025.

What questions do developers ask about regex debugging?

Common questions about regex flags, capture groups, catastrophic backtracking, and how this tool compares to production regex engines.

  • What is the difference between greedy and lazy quantifiers in regex?

    Greedy quantifiers (* + {n,}) match as many characters as possible while still allowing the overall pattern to succeed. Lazy quantifiers (*? +? {n,}?) match as few characters as possible. The visualizer shows greedy quantifiers as loops that consume maximally, while lazy ones show a preference for early exit.

  • Can the visualizer explain what my regex pattern does in plain English?

    Most visualizers include a "pattern explanation" mode that translates each part of the regex into a sentence. For example, \d{3}-\d{2}-\d{4} becomes "3 digits, a hyphen, 2 digits, a hyphen, 4 digits." This is generated from the parse tree, not hardcoded descriptions.

  • How do I test if my regex matches a specific string?

    Paste your regex into the pattern field and your test string into the test input field. The tool highlights matched portions and shows which capture groups extracted which substrings. If the pattern does not match, it shows where the match failed.

  • Does the visualizer support all regex flavors (JavaScript, Python, PCRE)?

    Most visualizers default to JavaScript regex syntax (the ECMAScript spec). Some support flavor switching to Python, PCRE, or .NET. The syntax differences are minor for common patterns (character classes, quantifiers) but significant for advanced features (lookbehinds, atomic groups). Check the tool's flavor selector.

  • What are capture groups and how do they work?

    Capture groups are parts of a regex pattern wrapped in parentheses: (\d+). The matched substring inside the group is saved and can be referenced later via backreferences (\1) or extracted programmatically. The visualizer numbers each group and shows which text each group captured.

  • Can I visualize multiline or case-insensitive regex patterns?

    Yes. The tool supports flags like /i (case-insensitive), /m (multiline), and /g (global). Enable these flags in the tool's flag selector, and the visualization updates to reflect the behavior change. For example, enabling /m makes ^ match the start of any line, not just the start of the string.

Related tools

These tools work alongside the Regex Visualizer for validating structured data and decoding encoded strings before pattern matching.

See what your regex actually matches

Regular expressions are powerful and hard to read. A pattern that looks fine can match too much, too little, or nothing. Testing it against real text is the only way to know.

Type a pattern, paste your sample text and watch the matches light up. You get the index of each match, the contents of every capture group, runtime stats and a preview of what a replacement would produce.

JavaScript flavor only

The tester uses the JavaScript regex engine. That's what runs in browsers and Node. Python, PCRE and other flavors mostly overlap, but they differ on features like lookbehind rules, named group syntax and flags. A pattern that works here may need small changes elsewhere.

The tool shows you matches, not a plain-English translation of the pattern. If you're unsure what part of a pattern does what, build it up in pieces and watch how the matches change.

Flags change everything

The g flag finds all matches instead of stopping at the first. The i flag ignores case. The m flag makes caret and dollar match at line breaks, which matters for multiline text. The s flag lets the dot match newlines too. Turn them on and off and see the difference right away.

Capture groups

Parentheses create a group. The text it matched is saved, and you can reuse it in a replacement with $1, $2 and so on. Named groups, written as (?<name>...), are easier to read in long patterns.

Use (?:...) when you need grouping for structure but don't need to save the text. It keeps the numbered groups tidy.

Greedy versus lazy

Quantifiers like * and + are greedy. They grab as much text as they can. Add a question mark and they become lazy, taking as little as possible. This matters a lot with delimiters. Run <.+> on a line with two HTML tags and it swallows everything between the first open bracket and the last close bracket. Run <.+?> and you get each tag.

When a regex matches more than you expected, greedy quantifiers are the first thing to check.

Slow patterns

Some patterns with nested quantifiers can take forever on certain input, because the engine tries too many paths. The runtime stat helps you catch that. If a test takes noticeably longer as the text grows, simplify the pattern before it reaches production.

Practice tips

Test with text that should match and text that shouldn't. Edge cases are where regexes break.

Anchor patterns with ^ and $ when you validate a whole value. Without them, a pattern can match a piece in the middle of bad input.

Don't parse HTML with regex. Use a parser.

Everything runs in your browser, so your sample text stays with you.