Developer Tools
JSON Formatter
Beautify, format, and validate JSON data with syntax highlighting.
Regex Tester
Test, debug, and analyze regular expressions client-side instantly. Features live match highlighting, flags selection (global, case-insensitive, multiline, dotAll), syntax validation with detailed error banners, and full breakdowns of match indices and captured groups.
Regular Expression
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Visual Highlights
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Security check: All regex evaluation runs locally in your browser. No data is sent to our servers.
What is the Regex Tester?
The FreeTechLearner Regex Tester is a real-time tool to write, test, and debug JavaScript regular expressions. It provides instant visual match highlighting, compiles regex safely to catch syntax errors, and extracts captured groups with full indices details.
How to Use
- Input your regular expression pattern in the top pattern bar.
- Configure flags: check/uncheck Global (g), Ignore Case (i), Multiline (m), or dotAll (s).
- Type or paste your test subject text into the Test String input panel.
- Check the highlighted preview to see matches, and look at the match list to review match indexes and captured groups.
- If an error occurs, look at the red warning banner for syntax details.
Common Use Cases
- Testing email, telephone, URL, or postal code validation regexes
- Debugging capture groups in complex string extraction patterns
- Validating matching behavior for multiline and case-insensitive text blocks
- Quickly checking replacement boundaries using zero-width matches
Key Benefits
- 100% browser-based execution for strict data privacy
- Real-time compile validation with precise syntax warnings
- Comprehensive breakdown of captured groups and indexes
- Handles zero-width matches safely without browser freezes
- Interactive match highlights with custom styles
Frequently Asked Questions
Which regex flavor does this use?
This tester uses the JavaScript regular expression engine (ECMAScript), which runs natively in your browser. Most patterns behave similarly to other engines like PCRE or Python, but some features (like lookbehinds) may depend on browser support.
What do the regex flags mean?
Flags modify how patterns are searched: "g" (Global) finds all matches in the text instead of stopping at the first; "i" (Ignore Case) makes matching case-insensitive; "m" (Multiline) allows ^ and $ to match start/end of lines; "s" (dotAll) makes the dot (.) match newline characters.
How do captured groups work?
Parentheses (e.g. (group)) define a capturing group. When a match is found, the tester extracts the content matched inside these parentheses and lists them under each match detail card, making parsing checks straightforward.
Does the tester handle zero-width matches safely?
Yes. Certain patterns (like ^, \b, or x*) can match zero-length character sequences. The tester is optimized to highlight these boundaries without causing browser infinite loops or freezes.
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