A regex tester is useful for checking a search or validation expression against representative text. Include both examples that should match and examples that should not.
Open the Regex Tester to follow the workflow. Processing runs in your browser; the tool does not upload the input for conversion.
A practical workflow
- Enter sample text and a pattern. Paste representative text. Enter the JavaScript pattern without slash delimiters.
- Set flags and replacement options. Use g for repeated matches; enable replacement only when you need a replacement preview.
- Review ranges and capture groups. Run the tool and inspect highlighted matches, offsets and captured values.
- Save the result or refine the pattern. Download the report. Simplify the pattern or shorten the sample if it times out.
Try a concrete example
The following demonstration input is also available in the workspace. Replace it with your own data after checking the expected behaviour.
Contact [email protected] or [email protected]Know the regex engine
This workspace uses JavaScript RegExp. Syntax and behaviour can differ from PHP PCRE, Python or other engines. Paste the pattern without surrounding slash delimiters, then enter flags separately. The default g flag finds successive matches. Other flags can change case sensitivity or multiline handling.
Read groups and offsets
Highlighted text shows matched ranges. The table reports offsets and numbered or named capture groups. JavaScript string offsets use UTF-16 code units, which matters for characters outside the basic multilingual plane. A zero-length match is shown with a visible marker so it is not mistaken for no match.
Keep difficult patterns bounded
Processing runs in a worker that is terminated if the two-second time budget is exceeded. More than 500 displayed matches are truncated; replacement preview asks for a smaller sample rather than silently pretending it covers everything. Simplify nested quantifiers or reduce the input if a pattern times out.
If the result needs attention
This tool uses JavaScript RegExp, not PCRE or Python syntax. Work runs in a cancellable worker with a timeout to prevent a difficult pattern freezing the page.
Read the exact error before changing the input. Keep a copy of the original, reduce the sample when investigating a large file, and use the destination application to verify that the result has the meaning you intended. The tool processes the supplied data; it cannot infer an unstated application schema or project requirement.
Use the Text Diff Checker to compare a saved result with the original. Keep transformation and application testing as separate steps so presentation changes are not mistaken for verified behaviour.
Sources and further reading
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