Regex Tester Online
Test and debug regular expressions in real-time. See matches highlighted instantly with group capture display — no server upload, fully private.
Matches
Capturing Groups
What Is a Regex Tester?
A regex tester is an interactive tool that helps developers write, test, and debug regular expressions (regex) in real-time. It shows exactly which parts of your input text match your pattern, making it invaluable for building and troubleshooting regex patterns for validation, search, parsing, and text transformation tasks.
Common Regex Patterns
\w+@\w+\.\w+ Phone (US)
\(\d3\)\s\d3-\d4 URL
https?://\S+ IP Address
\d3\.\d3\.\d3\.\d3 Understanding Regex Flags
g — Global
Find all matches, not just the first one.
i — Case-Insensitive
Ignore case when matching letters.
m — Multiline
^ and $ match line boundaries, not string boundaries.
s — Dotall
The . metacharacter matches newlines too.
Use Cases for Regex Testing
- Form Validation — Validate email addresses, phone numbers, URLs, and postal codes in web forms.
- Log Parsing — Extract structured data from server logs, application logs, and error reports.
- Data Extraction — Pull specific patterns from large text documents, API responses, or configuration files.
- Search & Replace — Find and transform text patterns across files with regex-based substitution.
- Input Sanitization — Filter and clean user input by matching allowed or disallowed patterns.
How to Use This Regex Tester
Enter your regex pattern between the forward slashes in the pattern input field. Add flags (g, i, m, s, u) using the flag buttons or the flags input. Type or paste your test string in the text area below. Matches are highlighted in real-time with colored backgrounds, and capturing groups are displayed in the groups section below the matches. If your regex is invalid, an error message will show the syntax problem.
Regex Tester FAQ
What is a regex tester used for?
A regex tester helps developers write, test, and debug regular expression patterns interactively. It shows real-time match results, captured groups, and highlights matches in the test string, making regex development much faster than trial-and-error coding.
Is this regex tester free?
Yes, completely free with no limits, no sign-up required, and no server uploads. All regex matching runs locally in your browser.
What regex flags does this tool support?
The tool supports g (global match), i (case-insensitive), m (multiline), s (dotall — . matches newlines), and u (unicode) flags. Click the flag buttons or type flags directly in the flags input.
What is a capturing group in regex?
A capturing group is a portion of a regex pattern enclosed in parentheses () that captures the matched substring for later use. Groups are numbered by the position of their opening parenthesis. This tool displays all captured groups for each match.
What regex engine does this use?
This tool uses JavaScript's built-in RegExp engine (ECMAScript specification). It supports most common features including quantifiers, character classes, groups, backreferences, lookaheads, and lookbehinds.
Is my data safe when testing regex?
Yes. Everything runs in your browser with no server communication. Your patterns and test data never leave your device.
What is the difference between greedy and lazy matching?
Greedy quantifiers (like .* or .+) match as much text as possible while still allowing the overall pattern to match. Lazy quantifiers (like .*? or .+?) match as little as possible. Adding a ? after a quantifier makes it lazy.
How do I use lookaheads and lookbehinds?
Lookaheads (?=pattern) assert what follows, while lookbehinds (?<=pattern) assert what precedes. Negative lookaheads (?!pattern) and lookbehinds (?<!pattern) assert the absence of a pattern. This tester supports all four types.