> ?#'a'
 => "#a"

> ?#
 => "#"

> ??
 => "?"

> ?..class
 => String

> self.method('?')
# NameError (undefined method `?' for class `#<Class:#<Object:0x08ce9188>>')

> ?1
 => "1"

> ?11
Traceback (most recent call last):
        1: from /home/prs/.rvm/rubies/ruby-2.5.3/bin/irb:11:in `<main>'
SyntaxError ((irb):33: syntax error, unexpected '?')

?a       #=> "a"
?abc     #=> SyntaxError
?n      #=> "n"