Edge Cases on Demand
Regex engines behave differently at boundaries. Our generator automatically creates strings that test anchors, empty inputs, Unicode boundaries, and quantifier extremes so you don't have to dream them up manually.
Regex Test Case Builder
Stop guessing whether your regex handles edge cases. Input any pattern and instantly generate a curated set of matching, non-matching, and boundary-pushing strings designed to expose logic flaws before they hit production.
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Generated Test Suite
Why Use a Test Case Builder?
Positive & Negative Pairs
Every test suite includes strings that should match and strings that should definitely not match. This dual approach catches both false positives and false negatives, the two most common regex bugs in production.
Instant JSON Export
Copy the entire test suite as a JSON array ready to paste into unit tests. Each entry includes the test string, expected result, and a reason describing what edge case it targets.
Flag-Aware Generation
Toggle case-insensitive, global, and multiline flags to see how they affect matching behavior. The generator adjusts its output to reflect real engine semantics for each flag combination.
How the Generator Builds Test Cases
- Parse the pattern – The tool extracts literals, character classes, quantifiers, anchors, and groups from your regex.
- Derive matching strings – It constructs valid strings that satisfy each component of the pattern, including boundary values for quantifiers like zero, one, and many.
- Derive non-matching strings – It mutates the pattern slightly to produce strings that violate character classes, miss required groups, or exceed length constraints.
- Apply flags – The selected flags modify the matching logic, and the generator recalculates expected results accordingly.
- Bundle and export – Results are displayed in a readable table and can be copied as JSON for direct use in test frameworks like Jest, pytest, or JUnit.