Tool overview
What is a Regex Tester?
A regex tester evaluates regular expressions against sample text and shows matches, capture groups, and flag behavior live.
Why use this Regex Tester?
Iterate on patterns for validation, parsing, or search without redeploying code—or leaking production strings to an online tester.
Key Features
Live match highlighting, capture groups, flag controls, and fully local JavaScript regex evaluation.
How to Use
Follow these steps to get accurate results from the tool interface above.
- Enter a JavaScript regular expression pattern in the pattern field (slashes are optional—the engine uses the RegExp constructor).
- Toggle flags (g, i, m, s, u, y) to control global search, case insensitivity, multiline ^/$, dotAll, Unicode, and sticky matching.
- Paste a test string into the subject input—matches highlight live with capture group tables below.
- Inspect SyntaxError messages for invalid escape sequences or unclosed character classes before debugging match logic.
- Copy match results or export capture groups once the tool launches with full clipboard integration.
JavaScript RegExp Syntax Reference
The tester uses the built-in ECMAScript RegExp engine. These patterns distinguish valid syntax from runtime exceptions.
| Feature / Context | Valid Syntax Example | Common Pitfall / Invalid Example |
|---|---|---|
| Character Class | [a-zA-Z0-9_]+ | [z-a]Invalid range endpoints throw SyntaxError at construction—reverse character ranges are not auto-corrected. |
| Quantifiers | a{2,4} | a{4,2}Min quantifier cannot exceed max—{4,2} fails parsing before any match attempt. |
| Escape Sequences | \d+\.\d+ | \xGGInvalid hex escapes in patterns throw SyntaxError; validate escape syntax separately from subject string content. |
| Lookahead | (?=foo) | (?<=unsupported)Lookbehind support depends on engine version—unsupported assertions fail at parse time on older runtimes. |
Frequently Asked Questions
Expandable answers for common debugging bottlenecks and data privacy questions.
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Official Documentation & References
Authoritative specifications and platform documentation for this utility.