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+|[-+*/])")
.Cast<Match>()
.Where(m=>m.Groups[1].Success)
.Select(m=>m.Groups[1].Value)
.ToArray();
Cheers
Andi