Well, you have three different kinds of characters:
¤Numbers (0,1,2...9)
¤Variables (Letters) (a,b,c...z)
¤Operators (+,-,*,/)
1)
Start from the beginning of the string, if it is a number or letter store it then go to 2). If its an operator, store it and goto 3).
2)
Search the rest of the string. If the first character is the same as you found in 1), store it and repeat from 2). If it's another kind of character, go to 3).
3)
Group the things you've stored in 1) and 2) into a string, then save it. Repeat from 1) with the rest of the string.
Repeat these steps until the string is empty.
