Block IDs and references need to follow the AsciiDoc pattern conventions.
They must start with a character, _ or : that can be followed by any character, number or _, :, -, ..
Blanks are not allowed. While IDs of style [[id]] don't allow for characters like ? and !, IDs in the style of [#id] do. This is confusing, therefore this plugin advocates to not use these characters in any ID.