Practical guide
How to use Regex Tester
Regex Tester provides a fast place to build and inspect JavaScript regular expressions. Matches are highlighted in context and listed with indexes and capture groups, which makes subtle pattern behavior easier to understand.
Step by step
- Enter a pattern without surrounding slash delimiters.
- Choose JavaScript flags such as g, i, m, s, u, or y and add representative test text.
- Run the test and inspect highlighted matches, indexes, and captured groups.
Example
Pattern: \btool\w* · Text: tools, toolkit, taskMatches: tools; toolkitHow it works
The pattern is compiled with the browser's JavaScript RegExp engine. Global matching is enabled for the result list so every occurrence can be shown, while other requested flags are preserved.
Important limitations
- JavaScript syntax and features can differ from PCRE, Python, Java, or .NET regex engines.
- A poorly designed expression can be slow on large or adversarial text.
Real workflows
When this tool is useful
Input validation
Develop a bounded pattern for identifiers, filenames, or form fields using representative valid and invalid examples.
Log extraction
Test capture groups for dates, request IDs, or status values before applying a pattern in a script or observability query.
Text cleanup
Preview every match in copied content before replacing it elsewhere, reducing the chance of deleting a wider region than intended.
Review the result
What to check before using the output
Test empty strings, long lines, unexpected Unicode, newline boundaries, and near-matches. Confirm whether the pattern is anchored and whether global, multiline, case-insensitive, or Unicode flags change the result.
- Include positive and negative samples.
- Inspect every capture group.
- Test long adversarial input.
- Confirm syntax in the destination regex engine.
Choose the right method
When another tool is better
Use a parser for nested or grammar-based formats such as HTML, JSON, programming languages, and complex addresses. A regular expression is most reliable for well-bounded lexical patterns.
Privacy and browser behavior
The pattern and sample stay in the tab. Replace production names, email addresses, tokens, and log payloads with realistic synthetic examples before sharing a test case.
Frequently asked questions
Should I include /pattern/ delimiters?
No. Enter only the pattern and place flags in the separate flags field.
Why does the pattern work elsewhere but not here?
The other environment may use a different regex engine or escaping convention.