How can I split string into array or variable without any delimiter?

You can use the Substring Extraction (see http://tldp.org/LDP/abs/html/string-manipulation.html[^]):

stringInput=12345678
str1=${stringInput:0:2} # 12
str2=${stringInput:2:2} # 34
str3=${stringInput:4:2} # 56
str4=${stringInput:6:2} # 78

If the input string has variable length use a loop (${#stringInput} returns the length of the string and use variables for the position and length parameters of the substring extraction).

[EDIT]
The above are for bash. But the link contains also examples using the UNIX expr command. But note that the expr command uses 1-based indexes.