I think I understand. chr will convert all values < 256 into a string having byte persuasion, since it can; and only encodes a string of UTF-8 persuasion if it has to.

Meanwhile, the regex engine is expecting a UTF-8-encoded string and assumes it is, rather than understanding that it has a character whose ordinal is correctly encoded for the kind of string it is. IOW, the regex engine is not respecting the persuasion of the input argument.

If the regex engine properly treated the input string as an abstract string of characters, regardless of how they were encoded, then it truely would not matter how chr decided to encode it.

I expected chr to always emit a UTF-8 string if utf8 was in effect.