Idiosyncratic Ruby: Symbolic Validations

When exactly don't you have to :"escape" a Ruby symbol?

Because this question is somehow related to the Ruby interpreter's internal usage of symbols, the rules are not the most obvious ones:

: + Identifier¹, optionally appended by !, ?, or = (→ methods)
:@ + Identifier¹ (→ instance variables)
:@@ + Identifier¹ (→ class variables)
:$ + Identifier¹ (→ global variables)
:$ + Single identifier¹ character or 0-9 (→ Perl-style special variables)
:$- + Single identifier¹ character or 0-9 (→ Ruby interpreter CLI options)
:!, :!=, :!~, :%, :&, :*, :+, :-, :/, :<, :>, :^, :`, :|, :~, :$!, :$", :$$, :$&, :$', :$*, :$+, :$,, :$., :$/, :$:, :$;, :$<, :$=, :$>, :$?, :$@, :$\, :$`, :$~, :**, :+@, :-@, :<<, :<=², :<=>², :==, :===, :=~, :>=, :>>, :[], :[]=

¹ Valid for identifiers: A-Z, a-z, 0-9, _, non-ASCII characters. Not allowed to start with 0-9.
² Example of a syntactical edge case that is not 100% clear

More Idiosyncratic Ruby