Regex Tester
Test regular expressions with live matching and highlighting.
This tool runs entirely in your browser. Nothing you type is uploaded, logged or stored, which makes it safe for keys, tokens and other sensitive values.
What Is Regex Tester?
A regular expression (regex) is a pattern that describes a set of strings. It is a compact, powerful notation for matching, validating, extracting, and replacing text, and it is supported by virtually every programming language, text editor, and search tool.
A regex is built from literal characters, which match themselves, and special metacharacters, which encode rules. Character classes match sets of characters, quantifiers control how many times something may repeat, anchors tie matches to positions, groups capture sub-expressions, and alternation offers a choice between patterns.
This Regex Tester lets you enter a pattern and test text, then see matches highlighted in real time. It runs entirely in your browser using JavaScript's built-in regular expression engine.
How to Use Regex Tester
- Enter your regex pattern in the Regex Pattern field (without the surrounding slashes).
- Toggle the Global, Case Insensitive, and Multiline flags as needed.
- Enter the text to test against in the Test Text field.
- Matches are highlighted instantly and the match count is shown.
- If the pattern is invalid, an error message explains the problem.
Features
- Live matching with highlighted results as you type.
- Toggleable flags: Global (g), Case Insensitive (i), and Multiline (m).
- Match count displayed alongside the highlighted output.
- Clear error messages for invalid patterns.
- Common pattern examples for quick reference.
- Fully client-side — no pattern or text is uploaded.
Common Use Cases
- Prototyping validation patterns for emails, phone numbers, URLs, and dates.
- Extracting data such as numbers, identifiers, or key-value pairs from text.
- Debugging a regex that is not matching as expected before shipping it.
- Learning how metacharacters and flags affect matching.
- Testing replacement logic by observing which substrings match.
How It Works
The tool compiles your pattern into a JavaScript RegExp object with the selected flags and runs it against the test text using matchAll. Each match is wrapped in a highlight so you can see exactly which substrings the pattern captures.
Flags change how matching behaves. The global flag finds all matches rather than stopping at the first. The case-insensitive flag ignores letter case. The multiline flag changes how the start and end anchors interact with line boundaries.
If the pattern cannot be compiled — for example, because of unbalanced parentheses — the browser reports a syntax error, which the tool displays so you can fix the pattern.
Security and Privacy Considerations
Be aware of ReDoS (Regular Expression Denial of Service). Patterns with nested quantifiers or overlapping alternatives can cause catastrophic backtracking on certain inputs, hanging the engine. Test patterns on realistic input and avoid ambiguous nested repetition.
Regex is not a security parser. Do not use regex alone to validate security-critical input such as HTML, nested structures, or complex file formats; use a proper parser instead.
All matching is local, so your pattern and text never leave your browser.
When to Use Regex Tester
- Prototyping validation patterns for emails, phone numbers, URLs, and dates.
- Extracting data such as numbers, identifiers, or key-value pairs from text.
- Debugging a regex that is not matching as expected before shipping it.
- Learning how metacharacters and flags affect matching.
When Not to Use Regex Tester
- Parsing nested structures such as HTML or XML — use a proper parser instead.
- Security-critical input validation on its own — combine with server-side validation, sanitization, and authorization.
- Patterns with nested or overlapping quantifiers on untrusted input without testing for ReDoS.
Practical Examples
- Form validation: prototype an email or phone pattern here, test it against valid and invalid examples, then ship the verified pattern to your application.
- Log extraction: write a pattern that captures request IDs or error codes from log lines and use the highlighted matches to confirm it captures exactly what you intend.
Comparison
- Regex tester vs regex explainer vs regex generator: the tester runs a pattern against text; the explainer breaks a pattern into tokens and describes each; the generator provides ready-made patterns for common validations. Use them together — generate a starting point, explain it, then test it.
- Regex tester vs a parser: the tester is ideal for flat text matching and extraction, but it is not a substitute for a parser on nested or structured documents.
Frequently Asked Questions
- Do I need to include the surrounding slashes in the pattern?
- No. Enter only the pattern itself; the tool adds the flags separately through the checkboxes.
- What does the Global flag do?
- It finds all matches in the text instead of stopping at the first one.
- Why does my pattern show an error?
- The most common causes are unbalanced parentheses or brackets, an unescaped special character, or an invalid quantifier. The error message from the browser explains the issue.
- What is ReDoS?
- Regular Expression Denial of Service happens when a pattern with nested or overlapping quantifiers causes catastrophic backtracking on certain input, hanging the engine. Avoid ambiguous nested repetition.
- Is my text uploaded?
- No. Matching happens entirely in your browser. Neither the pattern nor the text is sent to a server.