Practical usage guide
What this tool does
Regex Tester uses the JavaScript RegExp engine provided by your browser. It can list and highlight matches, replace text with capture-group references, or split input into a JSON array. This makes it useful for quick browser-compatible pattern experiments.
Regular-expression syntax varies across ecosystems. A pattern written for PCRE, .NET, Java, or Python may use constructs or flags that JavaScript does not support, so test with representative input from the real target environment.
Quick start
Enter the smallest useful pattern
Start with a literal or one group, then add boundaries, quantifiers, and alternatives incrementally.
Choose JavaScript flags
Common flags include i for case-insensitive, m for multiline anchors, s for dot-all, and u or v for Unicode behavior.
Select match, replace, or split
Use Matches to inspect indexes and groups, Replace to test $1 or named references, and Split to preview tokenization.
Common use cases
Prototype a validation rule
Test a bounded pattern against valid, invalid, empty, and edge-case samples before placing it in a form.
Extract values from logs
Capture IDs, dates, levels, or fields from a representative log fragment without uploading the text.
Preview a text rewrite
Use capture groups and replacement references to confirm a bulk rename or cleanup rule.
Limitations and important notes
Matching is global in this tool
The engine adds the g flag when it is omitted so all matches can be displayed. Replace and split therefore also operate globally.
Match display is capped at 500
The Matches view stops collecting after 500 results and marks the count with a plus sign. Narrow the pattern for detailed inspection.
Pathological patterns can freeze a tab
Nested ambiguous quantifiers may cause catastrophic backtracking. Avoid testing untrusted patterns against very long text in the main browser thread.