How to Test a Regular Expression Step by Step

Step-by-step

1. Write the pattern and decide which flags you need.
2. Paste sample text that includes values that should match and values that should not.
3. Run the test and check every line, not only the first match.
4. Add anchors ^ and $ if the whole value must match, not just a part of it.
5. Try edge cases: empty text, very long text, unusual characters and the examples that should fail.
6. Copy the pattern into your code and test it there, since regex engines differ slightly between languages.

Flags that change the result

The i flag ignores case. The m flag makes ^ and $ match at the start and end of each line. The s flag lets the dot match a new line. The g flag finds every match rather than the first. Check which flags your code will actually use.

Greedy and lazy matching

By default quantifiers such as * and + match as much as possible. In the text <b>one</b><b>two</b>, the pattern <b>.*</b> matches the whole string, while <b>.*?</b> matches each tag separately. Use the lazy form when you want the shortest match.

Patterns that can be slow

Nested quantifiers such as (a+)+ can take a very long time on input that almost matches, because the engine tries many combinations. Keep patterns simple, avoid overlapping alternatives, and limit the length of input you test in production.

Engines differ

Features such as lookbehind, named groups and Unicode handling vary between JavaScript, Python, PHP and others. The Regex Tester uses the JavaScript engine, so confirm the result in your own language.