MyScanner.cs

using System;

namespace Scanner
{
    class MyScanner
    {
        static public String Scan(String input)
        {
            MyScanner scanner = new MyScanner();

            foreach (var character in input)
                scanner.step(character);
            scanner.step((char)0);

            return scanner.result();
        }

        private enum state
        {
            space  = 0,

            num    = 1,
            var    = 2,
            add    = 3,
            sub    = 4,
            div    = 5,
            mul    = 6,
            bro    = 7, // BROOOOOO!!!!!!
            brc    = 8,

            end    = 999
        }

        private state currentState;
        private String res;
        private String buffer;

        private MyScanner()
        {
            currentState = state.space;
            res = "";
            buffer = "";
        }

        private string result()
        {
            return res;
        }

        private void step(char character)
        {
            state nextState = calculateState(character);

            // no more {var|num} condition
            if (((currentState == state.var) && (nextState != state.var)) || ((currentState == state.num) && (nextState != state.num)))
                res += String.Format("{0} - ({1})\n", (int)currentState, buffer);

            switch (nextState)
            {
                case state.add:
                case state.sub:
                case state.div:
                case state.mul:
                case state.bro:
                case state.brc:
                    res += String.Format("{0}\n", (int)nextState);
                    break;

                case state.num:
                case state.var:
                    if (currentState != nextState)
                        buffer = character.ToString();
                    else
                        buffer += character;
                    break;
            }

            currentState = nextState;
        }

        private state calculateState(char character)
        {
            if (Char.IsLetter(character))
                return state.var;
            if (Char.IsDigit(character))
                return (currentState == state.var) ? state.var : state.num;
            switch (character)
            {
                case '+': return state.add;
                case '-': return state.sub;
                case '*': return state.mul;
                case '/': return state.div;
                case '(': return state.bro;
                case ')': return state.brc;
                case ' ': case '\t': case '\n': return state.space;
                case (char)0: return state.end;
            }
            return state.end;
        }
    }
}