2. alnum Any alphanumeric character ("[A-Za-z0-9]").
####
my $re = qr{
\A # Assert start of string
[A-Za-z0-9] # Must start with one of these
(?: # Followed by either
[A-Za-z0-9_.-]*? # Zero or more of these
[A-Za-z0-9] # But ending with one of these
| # OR
# Nothing
)
\z # Assert end of string
}x;
##
##
my $re = qr{
\A # Assert start of string
[A-Z a-z 0-9] # Must start with one of these
(?: # Followed by either
[A-Z a-z 0-9 _ . -]*? # Zero or more of these
[A-Z a-z 0-9] # But ending with one of these
| # OR
# Nothing
)
\z # Assert end of string
}xx;