public class MyCalc
    {
        const int DELIM = 1;
        const int NUMBER = 2;
        
        string prog, token;
        int pr, token_type;

        public MyCalc()
        {
        }

        bool IsDelim(char c)
        {
            bool v = false;
            if (c == '+' || c == '-' || c == '*' || c == '/' || c == '(' || c == ')')
                v = true;
            return v;
        }

        bool IsWhite(char c)
        {
            bool v = false;
            if (c == ' ')
                v = true;
            return v;
        }

        bool IsDigit(char c)
        {
            bool v = false;
            
            v = char.IsDigit(c);
            if (!v)
                if (c == '.' || c == ',')
                    v = true;
            return v;
        }

        int GetToken()
        {
            token_type = 0;
            token = "";
            if (pr >= prog.Length)
            {
                token = "";
                token_type = DELIM;
            }
            else
            {
                while (IsWhite(prog[pr]))
                    pr += 1;
                if (IsDelim(prog[pr]))
                {
                    token = prog[pr].ToString();
                    pr += 1;
                    token_type = DELIM;
                }
                else if (IsDigit(prog[pr]))
                {
                    while(!IsDelim(prog[pr]))
                    {
                        token += prog[pr];
                        pr += 1;
                        if (pr >= prog.Length)
                            break;
                    }
                    token_type = NUMBER;
                }
            }
            return token_type;
        }

        Single ConvertStringToSingle(string s)
        {
            Single v = Single.MaxValue;
            Single tmpF;

            if (float.TryParse(s, System.Globalization.NumberStyles.Float, System.Globalization.CultureInfo.InvariantCulture, out tmpF))
                v = tmpF;
            else if (float.TryParse(s, System.Globalization.NumberStyles.Float, System.Globalization.CultureInfo.CurrentCulture, out tmpF))
                v = tmpF;

            return v;
        }

        Single Primitive()
        {
            Single v;
            if (token == "")
                token = "0";
            v = ConvertStringToSingle(token);
            GetToken();
            return v;
        }

        Single Unary(char o, Single r)
        {
            Single v = r;
            if (o == '-')
                v = r * -1;
            return v;
        }

        Single Arith(char op, Single r, Single h)
        {
            Single v = 0;
            switch (op)
            {
                case '-':
                    v = r - h;
                    break;
                case '+':
                    v = r + h;
                    break;
                case '/':
                    v = r / h;
                    break;
                case '*':
                    v = r * h;
                    break;
            }
            return v;
        }

        Single Level6()
        {
            Single v;
            if (token == "(" && token_type == DELIM)
            {
                GetToken();
                v = Level2();
                GetToken();
            }
            else
                v = Primitive();
            return v;
        }

        Single Level5()
        {
            string op = "";
            Single v;
            if (token_type == DELIM && (token == "+" || token == "-"))
            {
                op = token;
                GetToken();
            }
            v = Level6();
            if (op != "")
                v = Unary(op[0], v);
            return v;
        }

        Single Level4()
        {
            return Level5();
        }

        Single Level3()
        {
            Single v, hold;
            char op;
            v = Level4();
            while (true)
            {
                if (token == "")
                    break;
                op = token[0];
                if (!(op == '*' || op == '/'))
                    break;
                GetToken();
                hold = Level4();
                v = Arith(op, v, hold);
            }
            return v;
        }

        Single Level2()
        {
            Single v, hold;
            char op;
            v = Level3();
            while (true)
            {
                if (token == "")
                    break;
                op = token[0];
                if (!(op == '+' || op == '-'))
                    break;
                GetToken();
                hold = Level3();
                v = Arith(op, v, hold);
            }
            return v;
        }

        string GetABS(string s)
        {
            string v = s;
            string n, k = "";
            int i = s.IndexOf("ABS(", StringComparison.CurrentCultureIgnoreCase);
            if (i > -1)
            {
                if (i == 0)
                    n = "";
                else
                    n = s.Substring(0, i);
                string s1 = s.Substring(i + 3);
                int sc = 0;
                i = 0;
                int p = -1;
                while (i < s1.Length)
                {
                    if (s1[i] == '(')
                        sc += 1;
                    else if (s1[i] == ')')
                        sc -= 1;
                    if (sc == 0)
                    {
                        p = i;
                        break;
                    }
                    i += 1;
                }
                if (p > -1)
                {
                    k = s1.Substring(p + 1);
                    string s2 = s1.Substring(1, p - 1);
                    v = Math.Abs(Express(s2)).ToString();
                    k = GetABS(k);
                }
                v = n + v + k;
            }
            return v;
        }

        public Single Express(string s)
        {
            Single v;
            string s1;
            s1 = GetABS(s);
            pr = 0;
            prog = s1;
            GetToken();
            v = Level2();
            return v;
        }
    }

MyCalc cv = new MyCalc();
Single f = cv.Express("(1+2)/3");