Regex Tester
LiveFree online regex tester — test and debug regular expressions with real-time matching, highlighting, and capture group display. Alternative to RegExr.
Understanding Regular Expressions (Regex)
Regular expressions (regex) are pattern-matching sequences used throughout programming for input validation, text search, data extraction, URL routing, and log parsing. Every major language — JavaScript, Python, Java, Go, PHP, Ruby — implements regex engines with mostly compatible syntax. However, writing correct regex is notoriously tricky — quantifiers, anchors, character classes, lookaheads, and capture groups interact in subtle ways that make it easy to over-match, under-match, or create patterns vulnerable to catastrophic backtracking.
A free online regex tester and regular expression debugger. Paste your regex pattern and test string to see real-time match highlighting, capture groups, and flag results. Supports JavaScript regexp syntax with all flags — global (g), case-insensitive (i), multiline (m), dotAll (s), and unicode (u). Use it to build, test, and debug regular expressions interactively. View all matches with named and numbered capture groups, test replacement patterns, and get instant feedback as you type. Works as a powerful alternative to RegExr and other regex online tools. Whether you're writing a regex for validation, parsing, or search-and-replace — test it here instantly, 100% in your browser.
The Devkitr Regex Tester evaluates regular expressions against test strings in real time, highlighting all matches, showing capture group contents, and reporting match positions. Write a pattern, enter test strings, and see immediately which parts match and what each group captures — with support for all JavaScript regex features including named groups, lookbehind, and Unicode property escapes.
In a typical development workflow, Regex Tester becomes valuable whenever you need to free online regex tester. Whether you are working on a personal side project, maintaining production applications for a company, or collaborating with a distributed team across time zones, having a reliable browser-based inspection tool eliminates the need to install desktop software, write one-off scripts, or send data to third-party services that may log or retain your information. Since Regex Tester processes everything locally on your device, your data stays private and your workflow stays uninterrupted — open a browser tab, paste your input, get your result.
Key Features
Real-Time Match Highlighting
Matches are highlighted in the test string as you type the pattern, providing instant visual feedback on pattern accuracy.
Capture Group Display
Lists each capture group (numbered and named) with its matched content and position for every match occurrence in the test string.
Flag Toggles
Enable global (g), case-insensitive (i), multiline (m), dotAll (s), sticky (y), and unicode (u) flags with toggle buttons and see results change instantly.
Match Count and Positions
Shows total match count with start and end character positions for each match, useful for debugging substring extraction logic.
How to Use Regex Tester
Enter Your Pattern
Type a regular expression in the pattern field. Use the flag toggles to enable case-insensitive, global, multiline, or other modes.
Add Test Strings
Enter one or more test strings that represent the data you want to match — include both expected matches and non-matches.
Review Match Results
Highlighted matches and capture group contents appear in real time, showing what the pattern matches and captures.
Iterate and Refine
Modify the pattern and see results update instantly. Test edge cases, boundary conditions, and adversarial inputs.
Use Cases
Building Input Validation Patterns
Test email, phone number, URL, and custom format validators against a range of valid and invalid inputs to verify correct accept/reject behavior.
Extracting Data from Structured Text
Write patterns to extract specific fields from log lines, CSV rows, or API responses, using capture groups to isolate the data you need.
Developing Search and Replace Logic
Test find-and-replace patterns before applying them to codebases or text files, preventing unintended replacements in production data.
Debugging Failing Pattern Matches
When a regex doesn't match expected input, test it interactively to identify whether the issue is anchors, quantifiers, escaping, or flag settings.
Pro Tips
Start with the simplest pattern that matches your core case, then incrementally add complexity for edge cases — complex regexes written all at once are hard to debug.
Use named capture groups (?<name>...) instead of numbered groups for self-documenting patterns that are easier to maintain.
Always test with inputs that should NOT match to verify your pattern correctly rejects invalid data — positive test cases alone miss over-matching bugs.
Escape special regex characters (. * + ? ^ $ { } [ ] | ( ) \) when you want to match them literally — unescaped metacharacters are the most common regex bug.
Common Pitfalls
Not escaping dots when matching literal periods
Fix: An unescaped . matches any character. Use \. to match a literal period — e.g., in domain validation, user\.name@domain\.com.
Using greedy quantifiers (.* ) when lazy (.*?) is needed
Fix: Greedy .* matches as much as possible, often consuming content you wanted to capture in later groups. Use .*? for shortest possible match.
Forgetting anchors (^ and $) for full-string validation
Fix: Without anchors, a validation pattern like \d{3} matches "abc123def". Add ^\d{3}$ to ensure the entire string is exactly 3 digits.
Frequently Asked Questions
QWhat is regex (regular expression)?
A regex (regular expression) is a pattern used to match, search, and manipulate text. Regular expressions are used in programming, text editors, and command-line tools for tasks like validation, parsing, and find-and-replace operations.
QHow do I test a regex online?
Enter your regex pattern in the top field and your test string below. Matches are highlighted in real-time as you type. All capture groups, match indexes, and flag results are displayed instantly.
QWhich regex flavor is used?
The tool uses JavaScript's built-in RegExp engine, which supports most common regex features including lookaheads, lookbehinds, named capture groups, and unicode properties.
QHow is this different from RegExr?
This regex tester runs 100% in your browser with no signup or tracking. It provides the same real-time regex matching and highlighting as RegExr, plus capture group display and flag toggling — all in a clean, distraction-free interface.
QCan I see capture groups?
Yes. All matched groups — both numbered and named capture groups — are displayed with their values for each match, making it easy to debug complex regular expressions.
QWhich flags are supported?
Global (g), case-insensitive (i), multiline (m), dotAll (s), unicode (u), and sticky (y) flags are all supported. Toggle them with a single click.
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