The optional half

Match a username, optionally followed by an @ and a host. Capture the username as group one and the host as group two. When there is no host, group two must take no part at all — not match an empty string.

Corpus

Steps0/ 603 · par 201

Every input, what the pattern matched in it, and what each group captured
Input	Match	Steps
	(empty)	3
	(empty)	3
	(empty)	3
	(empty)	3
	(empty)	3
	(empty)	3

Trace

Select an input to step through what your pattern did to it.

Cmd-Enter submits too.