Regular expressions blacklist everything but upcase chars 'string' !~ /[^A-Z]/ blacklist everything but case-insensitive chars 'string' !~ /[^a-z]/i blacklist everything but case-insensitive chars, numbers and whitespace 'string' !~ /[^a-z0-9]\s/i Operators .eql? versus == 1 == 1.0 #=> true 1.eql? 1.0 #=> false