Hex escape sequences are taking many hex digits, not just two.
Your original sample was just a string with 1 character (with a very long syntax) and a zero byte.
Don't use hex escape sequences if the next character maybe taken as hex digit. The use of 3 octal digits is safe ("\0251234567890") or you can terminate one string literal and start a new one ("\x15" "1234567890")
or, and that's probably best, use one of the ways Lundin suggested.
Daniel