RegEx Tester

Test and debug regular expressions with instant visual feedback

Web Dev Tools

Tip: Enter your regex pattern and test string. Matches are highlighted in real-time. Click on common patterns for quick insertion.

Quick Patterns

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What the regex tester does

A regular expression is a pattern string that matches, searches, or replaces text. This regex tester takes your pattern, applies the flags you select, and runs it against your test text using the browser's native RegExp engine. Matching happens on your device, so no text or patterns leave your computer.

As you type, the tool highlights every match in the test text and updates a match counter. It reads the pattern, compiles it once per keystroke, and shows the results area with details for each hit, including any capture groups you define with parentheses.

Why real-time matching matters

Regex mistakes are hard to spot by reading alone. A missing anchor or a greedy quantifier can silently match too much or nothing at all. Live highlighting exposes that behavior the moment it happens, so you correct the pattern before it reaches your code.

Because the work runs client-side, there is no round trip to a server between edits. That keeps feedback tied directly to your keystrokes. The main benefits of testing this way:

You see exactly which substrings match and which do not.
The match counter confirms whether a pattern hits the expected number of times.
Capture groups appear in the results, so you can verify extraction logic.
Nothing is uploaded, so sensitive sample data stays local.

How to test a pattern step by step

The workflow is built around the pattern input, the flag toggles, and the test text area. Each control feeds the same engine, and any change recompiles the expression.

Open the RegEx Tester from Web Dev Tools.
Type your regular expression in the pattern input field.
Toggle the flags you need: g (global), i (case-insensitive), m (multiline), s (dotAll), u (unicode), or y (sticky).
Paste or type your test text in the textarea below.
Read the highlighted matches, check the counter, and review match details in the results area.

If you want a starting point, the Quick Patterns row inserts ready-made expressions. Click Numbers, Words, Letters, Email, URL, or Hex Color, and the pattern drops into the input for you to adjust against your own text.

Quick patterns and common cases

The built-in patterns cover tasks people write over and over. Each button writes a tested expression into the input, which acts as a small regex generator for routine jobs. From there you edit freely.

Numbers and Words for splitting or counting tokens.
Letters for alphabetic-only checks.
Email and URL for validating input formats.
Hex Color for matching color codes in stylesheets.

Use these as templates rather than final answers. A validation pattern that fits one dataset may need tightening for another, and editing a working example is often clearer than starting from a blank field.

How this online regex tester differs

This tool uses JavaScript's RegExp engine, which follows the ECMAScript flavor. That flavor differs from PCRE, Python's re , and Java in a few places, so a pattern verified here may behave differently elsewhere.

Some lookbehind and named-group syntax varies between flavors.
Flag names and defaults are not identical across languages.
Certain Unicode property escapes require the u flag here.

If you plan to port a pattern into Java, treat this as an online regex tester for the logic, then confirm escaping rules in your target runtime. When you need a starting expression from a plain description, the regex generator can draft one you then refine here. For comparing two blocks of matched output, the diff checker helps.

When to reach for this tool

Reach for a regex checker whenever a pattern touches real data and you cannot afford a wrong match. Common moments include:

Writing validation for forms or config files.
Building search-and-replace rules for logs or source code.
Debugging a pattern that matches too much or too little.
Learning regex syntax by watching highlights change as you edit.

For broader text and data work, the DevSwitch tool collection covers converters, formatters, and encoders alongside this one.

FAQ

It is a tool that runs a regular expression against sample text and shows the results. You enter a pattern, choose flags, and paste text. The tool compiles the pattern with the browser's engine and highlights each match while a counter reports how many were found. This lets you confirm the pattern behaves as intended before you use it in code.

Every time you edit the pattern, the flags, or the test text, the tool recompiles the expression using JavaScript's native RegExp engine and scans the text again. Matches are located, highlighted, and counted in the results area. All of this runs in your browser, so results appear as you type without contacting a server.

No. Matching happens entirely in your browser with client-side JavaScript. Your pattern and your test text are never transmitted to any server. This means you can paste production data, log samples, or private strings without them leaving your device, and the tool keeps working even if you go offline after loading the page.

Flags modify how the pattern runs. The g flag finds all matches instead of stopping at the first. The i flag ignores case. The m flag makes anchors match line by line. The s flag lets the dot match newlines. The u flag enables full unicode handling, and y anchors matching to a fixed position.

A capture group is a part of the pattern wrapped in parentheses. It extracts a sub-match from within a larger match, which is useful when you want a specific piece such as a date's year or a URL's domain. The results area shows captured values, so you can confirm the group grabs exactly the portion you expect.

You can test the core logic here, but be careful. This tool uses JavaScript's ECMAScript flavor, while Java uses its own engine. Character classes and simple quantifiers behave the same, but lookbehind, named groups, and some escapes differ. Build and check your pattern here, then verify escaping and syntax in a Java environment before shipping it, since string escaping rules also differ.

Quick Patterns is a row of buttons for common tasks: Numbers, Words, Letters, Email, URL, and Hex Color. Clicking a button inserts a working expression into the pattern input, ready to run against your text. They act as a small pattern library and a starting point. You can edit the inserted pattern to match your exact requirements.

They solve different halves of the same job. The regex generator helps you produce a pattern from a description or example. This tester takes an existing pattern, runs it against sample text with the engine, and shows highlighted matches and groups. A common flow is to draft a pattern in the generator, then paste it here to confirm it matches your real data.

Usually the pattern is stricter than the text or a flag is missing. Anchors like ^ and $ tie a match to line boundaries, so multiline text may need the m flag. Case differences need the i flag. Special characters such as dots and brackets must be escaped with a backslash when you mean them literally. The counter reading zero confirms the mismatch.

There is no fixed cap, but practical limits come from your device. Because matching runs in the browser, very large text combined with a complex pattern can slow highlighting or cause catastrophic backtracking on poorly written expressions. If things stall, shorten the sample or simplify the pattern. For most validation and debugging work, typical text sizes run without noticeable delay.

No. This is a browser-based regex online tool. You open the page and start typing. All processing uses the browser's built-in engine, so there is nothing to download, no account to create, and no extension to add. The page works on desktop and mobile browsers alike, and once loaded it continues to match text without a network connection.

This tool focuses on matching, highlighting, and reporting how many times a pattern hits, along with any capture groups. That coverage lets you confirm which parts of the text a replacement would touch before you run it in your own code. Verifying the match is the step that prevents a bad replacement, since the groups you capture become the pieces you reference in a substitution.