Regex Tester

Why a live tester beats writing regex blind

Regular expressions are notoriously unforgiving — a misplaced backslash or forgotten anchor can silently change what a pattern matches. Writing them directly into production code and then running the program to see what happens is slow and error-prone. A live tester shortens the feedback loop to zero: you see what matches the moment you type, which makes it realistic to iterate toward the exact pattern you want.

The flags you will actually use

• g (global) — find all matches, not just the first. This is almost always what you want for search operations.
• i (case-insensitive) — ignore case. Essential for matching user input.
• m (multiline) — ^ and $ match line starts/ends, not just the whole string. Useful for log parsing.
• s (dotall) — . matches newlines as well. Helpful when you want to match across lines.
• u (unicode) — treat the pattern as a Unicode code point sequence. Required for emoji and astral characters.

Capture groups — the part most people underuse

Any parentheses in your pattern create a capture group. This tool shows every group for every match, letting you extract specific parts of matched text — the username from an email, the path from a URL, the version number from a tag. Named capture groups ((?<name>...)) also work and are often easier to read than numbered ones.

Limitations to know

This tester uses the JavaScript regex engine, which differs from Python, Go, Ruby, PCRE, and .NET flavors in subtle but important ways. Lookbehind assertions, for example, behave differently across engines. If you are writing a pattern for use outside JavaScript, sanity-check it in your target language.