How do I test a WHOLE string against regex in ruby?

You can match the beginning of the string with \A and the end with \Z. In ruby ^ and $ match also the beginning and end of the line, respectively:

>> "a\na" =~ /^a$/
=> 0
>> "a\na" =~ /\Aa\Z/
=> nil
>> "a\na" =~ /\Aa\na\Z/
=> 0