reference            ::= name (":" tag )? ("@" digest )?
name                 ::= (domain "/")? path
domain               ::= host (":" port-number)?
host                 ::= domain-name | IPv4address | "[" IPv6address "]" /* see https://www.rfc-editor.org/rfc/rfc3986#appendix-A */
domain-name          ::= domain-component ("." domain-component)*
domain-component     ::= ([a-zA-Z0-9]|[a-zA-Z0-9][a-zA-Z0-9-]*[a-zA-Z0-9])
port-number          ::= [0-9]+
path-component       ::= [a-z0-9]+ (separator [a-z0-9]+)*
path                 ::= path-component ("/" path-component)*
separator            ::= [_.] | "__" | "-"+

tag                  ::= [\w][\w.-]{0,127}

digest               ::= algorithm ":" encoded
algorithm            ::= algorithm-component (algorithm-separator algorithm-component)*
algorithm-separator  ::= [+._-]
algorithm-component  ::= [A-Za-z][A-Za-z0-9]*
encoded              ::= [a-fA-F0-9]{32,} /* At least 128 bit digest value */

identifier           ::= [a-f0-9]{64}