in reply to
Maybe Escape in Dynamic RegExps
This does what you want.
my $re = qr/(?<!\Q$c\E)\Q$d\E/;
The \Q . . \E pair is like quotemeta. I've changed the names to protect sort. Extra ( . . ) delimiter pair is cut.
The same name on every site (unless it is taken already)
A different name for every site
The same name for sites I don't care about, but different names on sites I do care about
Different names for sites I don't care about, but the same name on sites I do care about
The anonymous account
Someone else's username
Root, after hacking into the site
Results (218 votes),