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;