Regex Tester

JavaScript regex semantics and safe testing

The tester uses the regular-expression behavior provided by the current JavaScript engine. A pattern that works here may differ in Python, Java, .NET, PCRE, RE2, or a database. The page is designed for short representative samples and highlights matches; it is not a production safety analyzer.

Methodology

1. The entered pattern and flags are passed to the JavaScript RegExp constructor.
2. Matches are evaluated against the supplied sample with the engine’s Unicode, multiline, dotAll, case, sticky, and global behavior as selected.
3. Match locations and captured text are displayed for inspection.
4. Syntax errors are surfaced directly so an invalid expression is not silently treated as no match.

Worked review example

The pattern ^\d+$ checks an entire ASCII-digit sample only when anchors and the intended multiline behavior are understood. An email-shaped regex can demonstrate structure but should not replace confirmation mail or standards-aware validation. Before deployment, test accepted, rejected, empty, long, Unicode, and adversarial inputs in the same runtime that will execute the pattern.

Test coverage and expected behavior

Test coverage includes literals, groups, alternation, anchors, global matching, Unicode flags, zero-length matches, invalid syntax, and multiline samples. We also use nested-quantifier examples to confirm that the page does not describe a successful match as proof of safe runtime behavior.

Decision checklist before using the output

Copy the exact runtime and flags into the test plan. Build a table of inputs that must match, must not match, may be empty, contain Unicode, and approach the maximum expected length. Add adversarial near-matches because they often trigger much slower backtracking than successful examples. Anchor the pattern when the entire value must conform, and avoid using a regex alone to validate ownership, reachability, dates, numeric ranges, or complex standardized addresses. Measure performance in the production engine under realistic limits. If patterns can come from users, use a safe engine, restricted syntax, timeouts, or isolation rather than executing arbitrary JavaScript regexes on the main thread. The browser tester is useful for explaining captures and flags, but a green sample is only one observation; deployment requires negative tests and resource controls.

Important limitations

• Catastrophic backtracking can freeze a JavaScript thread on crafted input.
• Regex flavor differences affect escapes, lookbehind, Unicode properties, and flags.
• Matching a format does not verify ownership, existence, or business validity.
• Never run an untrusted pattern against large production input without controls.

Sources and specifications

These references define the relevant format or browser behavior. They do not endorse WordCaseFix.

• ECMAScript RegExp specification
• OWASP ReDoS guidance

Last reviewed: August 6, 2026