You ask about tokenizing
] your input string.
You can either take a tool to tokenize (see link above for some tools), or you can write your own tokenizer.
Writing your own tokenizer is either by using the C# Regex Class
] or by fully hand-craft it yourself.
For such simple tasks like this one, I usually use Regex, first since I know it well and secondly it is easy, if you know Regex ;-)
Your language is tokenized as follows (ignoring white spaces and non-matching characters):
string s = "...";
string tokens = Regex.Matches(s, @"(\d+|[-+*/])")