Question 1

Which regex flavor does this use?

Accepted Answer

This tool runs your pattern through JavaScript's native RegExp engine — the same one used in browsers and Node.js. Most core syntax (character classes, quantifiers, groups, lookahead) is shared across regex flavors, but some features differ from PCRE (used by PHP, many text editors) or Python's re module, so a pattern that works here should be spot-checked if you're deploying it in a different language.

Question 2

What do the i, m, and s flags do?

Accepted Answer

"i" makes matching case-insensitive. "m" changes ^ and $ to match the start and end of each line rather than the whole string, useful for multi-line input. "s" (dotAll) makes the dot (.) also match newline characters, which it excludes by default. The global flag (g) is always applied automatically so every match is found, not just the first.

Question 3

Why does my pattern match more or less than I expected?

Accepted Answer

The most common cause is a quantifier being "greedy" by default — .* will match as much as possible, sometimes spanning further than intended, where .*? (adding a ?) matches as little as possible instead. Unescaped special characters like ., *, or ( inside a pattern that should be literal are another frequent cause; escape them with a backslash to match them literally.

Question 4

What are capture groups, and why does the match list show them separately?

Accepted Answer

Parentheses in a pattern create a capture group — a portion of the match you can reference separately, commonly used to extract just the useful part of a larger match (like pulling a domain out of an email address). The match list shows the full match text alongside any captured groups from that same match, in the order they appear in the pattern.

Question 5

Is my test string or pattern sent anywhere?

Accepted Answer

No — pattern matching runs using your browser's built-in regex engine, entirely on your device. Nothing you type here is uploaded or logged, which also means it's safe to test patterns against real (if sensitive) sample data without it leaving your machine.

#	Match	Index	Groups
1	hello@calcuforge.com	14	—
2	support@example.com.	38	—

Pattern piece	Matches
[\w.+-]+	The local part before @
[\w-]+	The domain name
\.[\w.-]+	The dot and extension

Regex Tester & Visualizer

Common pattern building blocks

Extracting email addresses from text

Common questions