Allow regular expressions after ": "
Don't highlight HEREDOC in singleton class definition (operator <<)
Now, after the keyword "class", Heredoc's will not be highlighted . For example:
Before, <<Foo was highlighted as Heredoc.
I did some tests in Ruby 2.6.0, 2.5.1 and 2.3.8, and Heredoc isn't considered in the code: class <<Foo, class <<21, class <<"Foo" or class <<-Foo, so I can say that the fix is correct.
Improvements in HEREDOC:
- Add squiggly HEREDOC (introduced in Ruby 2.3) 
For example: <<~HEREDOC
- Backticks can surround identifiers . Example: <<-HEREDOC
- Improve highlighting of identifier quotes. Now you need to close quotes to highlight the HEREDOC.
For example, HEREDOC was previously highlighted as: <<"HEREDOC , <<HEREDOC"
I also added a modeline to force the use of tabs as an indentation, since the file uses tabs.
-  Ruby/Rails Programming. Singleton Classes in Ruby (aka eigenclasses): https://codequizzes.wordpress.com/2014/04/11/singleton-classes-in-ruby-aka-eigenclasses/
-  Infinum. Multiline strings in Ruby 2.3 - the squiggly heredoc: https://infinum.co/the-capsized-eight/multiline-strings-ruby-2-3-0-the-squiggly-heredoc
-  Ruby-doc.org. Here Documents: https://ruby-doc.org/core-2.5.1/doc/syntax/literals_rdoc.html#label-Here+Documents