Regex Tester

Frequently Asked Questions

What regex flags can I use?

Common flags are g (global), i (ignore case), m (multiline), and s (dotAll). Combine them like gi. The engine is the standard JavaScript RegExp.

Why does only the first match show?

You likely left out the g (global) flag. Without it, only the first match is returned. Add g to find every match in the text.

What is a capture group?

Text inside parentheses is captured separately so you can extract parts of each match. This tester lists every captured group per match.

What happens if my pattern is invalid?

The tester catches the error and shows a clear message instead of crashing, so you can fix the syntax and try again.

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How This Calculator Works

A regular expression (regex) is a pattern used to search and match text. Enter a pattern, any flags such as g for global or i for case-insensitive, and a block of sample text. When you click Test, the tool builds the expression with the browser's RegExp engine and lists every match it finds, along with the position of each match and the contents of any capture groups.

How It Works

The pattern and flags are passed to new RegExp(pattern, flags) inside a guard. If the syntax is invalid, you get a clear error message instead of a crash. When the global flag is set, matchAll() finds every match in the text; without it, only the first match is returned. Each match reports its starting index (zero-based) and the text captured by parenthesised groups.

Common Mistakes

Forgetting the g flag and wondering why only the first match appears.

Putting slashes around the pattern; enter just the body, not /pattern/.

Not escaping special characters like the dot, so the dot matches any character instead of a literal period.

Using flags the engine does not support, which produces an invalid-expression error.

Tips & FAQ

What flags can I use? Common ones are g (global), i (ignore case), m (multiline), and s (dotAll). Combine them like gi.

What is a capture group? Text inside parentheses is captured separately so you can pull out parts of each match; this tool lists them per match.

Why does my pattern not match? Check for unescaped special characters and confirm your flags; the dot, plus, and question mark all have special meaning.

Is my text sent anywhere? No. The matching runs entirely in your browser with the built-in RegExp engine.

Frequently Asked Questions

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