How to properly add hex escapes into a string-literal?

Use 3 octal digits:

char problem[] = "abc\022e";

or split your string:

char problem[] = "abc\x12" "e";

Why these work:

• Unlike hex escapes, standard defines 3 digits as maximum amount for octal escape.

  6.4.4.4 Character constants

  …

  octal-escape-sequence:
      \ octal-digit
      \ octal-digit octal-digit
      \ octal-digit octal-digit octal-digit
  

  …

  hexadecimal-escape-sequence:
      \x hexadecimal-digit
      hexadecimal-escape-sequence hexadecimal-digit
  

• String literal concatenation is defined as a later translation phase than literal escape character conversion.

  5.1.1.2 Translation phases

  …

  5. Each source character set member and escape sequence in character constants and
     string literals is converted to the corresponding member of the execution character
     set; if there is no corresponding member, it is converted to an implementation-
     defined member other than the null (wide) character. 8)

  6. Adjacent string literal tokens are concatenated.