Start with representative examples
A regex is only as reliable as the examples used to test it. Include strings that should match, strings that should not match, and edge cases such as empty values, extra whitespace, different casing, punctuation, and multiline input.
Understand the flags
The global flag finds multiple matches, the case-insensitive flag ignores letter casing, and the multiline flag changes how line anchors behave. Test flags deliberately because changing one flag can alter results across a full log file or pasted document.
Review before copying into code
After a pattern works in the tester, copy it into the target language and run a small unit test there. Different languages and escaping rules can change how a pattern is written, especially when the regex lives inside a string literal.
How to use this guide
- 1
Add representative text
Include examples that should match, examples that should fail, and edge cases with whitespace, casing, punctuation, or multiline input.
- 2
Enter the pattern and flags
Test the regex with the intended g, i, and m flags instead of relying on defaults.
- 3
Port the pattern carefully
Copy the pattern into the target language and add a small unit test to catch escaping or runtime differences.
Frequently asked questions
Why should regex tests include non-matching examples?
Non-matching examples reveal whether a pattern is too broad. They are often better at finding brittle text matching logic than happy-path samples.
What do the g, i, and m flags change?
The g flag finds multiple matches, i ignores casing, and m changes how line anchors behave across multiline text.
Can I copy a tested regex directly into code?
Review escaping rules in the target language first. A pattern that works in a tester may need extra escaping inside a string literal.