A regex tester lets you compare a pattern with representative strings before using it in code. Test both matches and non-matches: a pattern that accepts one desired example may still accept too much or miss important edge cases.
Build a useful set of test cases
Write down the behavior you want in plain language first. Include ordinary examples, boundary cases, and strings that should be rejected.
- Enter the regular expression and any required flags.
- Add several positive examples and expected non-matches.
- Inspect full matches and capture groups.
- Refine the pattern, then retest the full set.
Keep in mind: Avoid using a complex regex as the only validation for structured formats such as email addresses or URLs when a dedicated parser is available.