Additional characters should be valid for the DOI id
The current validation process of a DOI id is as follows according to spec:
DOI Format (HP05) DOIs must be composed by a valid prefix, a slash ("/") and a suffix. A DOI (suffix) must be composed by the following characters only: 0-9 | a-z | A-Z | - (dash) | . (dot) | _ (underscore) | + (plus) | : (colon) | / (slash) DOIs are by definition case-insensitive. The submission of two DOIs that differ only in the case of the used characters will be rejected.
Validation should be change as follows: All characters should be allowed except: "?" | "@" | "&" | "=" | "+" | "$" | ","