Re^2: Match nothing in grep { ! // }

[bart]

The empty regex always matches

Well, yes, and no. // is special, it serves as a shortcut, as if it's not the first regex you match, it signifies a repeat of the last regex that matched.

perlreref:

If 'pattern' is an empty string, the last successfully matched regex is used.

In your tests here, there was no previous match, so this rule doesn't apply.