Finding chars in strings

[Per Nordlöw via Digitalmars-d-learn]

On Tuesday, 5 September 2017 at 15:43:02 UTC, Per Nordlöw wrote:
> If a character literal has type char, always below 128, can we 
> always search for it's first byte offset in a string without 
> decoding the string to a range of dchars?

Follow up question: If a character literal has type char, can we 
always assume it's an ASCII character?