PCRE semantics
                         │
                   compiled program
                         │
                 backtracking VM
                         │
                ┌────────┴────────┐
                │                 │
          UTF-8 subject      UTF-16 subject
         ReadOnlySpan<byte>  ReadOnlySpan<char>
                │                 │
           byte cursor       char cursor
                │                 │
                └──────┬──────────┘
                       │
                 Unicode scalar
                    decoder
                       │
                     Rune