For the first few years of Crossrefâs existence, the characters that were allowed in DOI suffixes were much more expansive. Over time, we found that certain kinds of characters (for example, angle brackets, colons, question marks, hash/pound signs) were causing problems for certain linking and query systems.
So, eventually suffix characters were limited to just A-Z, a-z, 0-9 and the following special characters: - . _ ; ( ) /