[22]    prolog      ::=    XMLDecl? Misc* (doctypedecl Misc*)?
[23]    XMLDecl     ::=    '<?xml' VersionInfo EncodingDecl? SDDecl? S? '?>'
[24]    VersionInfo ::=    S 'version' Eq ("'" VersionNum "'" | '"' VersionNum '"')
[25]    Eq          ::=    S? '=' S?
[26]    VersionNum  ::=    ([a-zA-Z0-9_.:] | '-')+
[27]    Misc        ::=    Comment | PI | S

• '+' means 'one or more', '?' means 'optional', '*' means 'zero or more'.

• Parenthesis group constructs.

• '|' (pipe character) means 'or'

• 'string' means the occurrence of the literal string.

• [c-c] is a character class and represents a single character in the specified range.