$ ruby -v ruby 1.9.3p484 (2013-11-22 revision 43786) [x86_64-linux] $ ruby symbol_literals.rb valid as first char: @$_ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz valid as middle char: _ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789 valid as end char: !_=?ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789 valid as a single character (eval):1: warning: invalid character syntax; use ?\s `~!%^&*-_+|<>/ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz $ # Note the eval warning relates to the disallowed symbol of :?