```ruby
module Treetop
  module Compiler    
    class CharacterClass < AtomicExpression
      def compile(address, builder, parent_expression = nil)
        super
        
        builder.if__ "input.index(Regexp.new(#{single_quote(text_value)}), index) == index" do
          assign_result "instantiate_node(#{node_class_name},input, index...(index + 1))"
          extend_result_with_inline_module
          builder << "@index += 1"
        end
        builder.else_ do
          "terminal_parse_failure(#{single_quote(characters)})"
          assign_result 'nil'
        end
      end
    end
  end
end
```

Use \G to force Regexps to match only at the first position
user
```ruby
module Treetop
  module Compiler    
    class CharacterClass < AtomicExpression
      def compile(address, builder, parent_expression = nil)
        super
        
        builder.if__ "input.index(Regexp.new(#{grounded_regexp(text_value)}), index) == index" do
          assign_result "instantiate_node(#{node_class_name},input, index...(index + 1))"
          extend_result_with_inline_module
          builder << "@index += 1"
        end
        builder.else_ do
          "terminal_parse_failure(#{single_quote(characters)})"
          assign_result 'nil'
        end
      end

      def grounded_regexp(string)
        # Double any backslashes, then backslash any single-quotes:
        "'\\G#{string.gsub(/\\/) { '\\\\' }.gsub(/'/) { "\\'"}}'"
      end
    end
  end
end
```
assistant