All the standard says on the matter is that you get at least the 52 upper- and lower-case latin alphabet characters, the digits 0 to 9, the symbols ! " # % & ' ( ) * + , - . / :
; < = > ? [ ] ^ _ { | } ~
, and the space character, and control characters representing horizontal tab, vertical tab, and form feed.
The only thing it says about numeric encoding is that all of the above fits in one byte, and that the value of each digit after zero is one greater that the value of the previous one.
The actual encoding is probably inherited from your locale settings. Probably something ASCII-compatible.
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