How can tokenize this string in java?

Question

How can I split these simple mathematical expressions into seperate strings?

I know that I basically want to use the regular expression: \"[0-9]+|[*+-^()]\"

Answer 1

Here is my tokenizer solution that allows for negative numbers (unary).

So far it has been doing everything I needed it to:

private static List tokenize(String expression)
    {
        char c;
        List tokens = new ArrayList();
        String previousToken = null;
        int i = 0;
        while(i < expression.length())
        {
            c = expression.charAt(i);
            StringBuilder currentToken = new StringBuilder();

            if (c == ' ' || c == '\t') // Matched Whitespace - Skip Whitespace
            {
                i++;
            }
            else if (c == '-' && (previousToken == null || isOperator(previousToken)) && 
                    ((i+1) < expression.length() && Character.isDigit(expression.charAt((i+1))))) // Matched Negative Number - Add token to list
            {
                currentToken.append(expression.charAt(i));
                i++;
                while(i < expression.length() && Character.isDigit(expression.charAt(i)))
                {
                    currentToken.append(expression.charAt(i));
                    i++;
                }   
            }
            else if (Character.isDigit(c)) // Matched Number - Add to token list
            {
                while(i < expression.length() && Character.isDigit(expression.charAt(i)))
                {
                    currentToken.append(expression.charAt(i));
                    i++;
                }
            }
            else if (c == '+' || c == '*' || c == '/' || c == '^' || c == '-') // Matched Operator - Add to token list
            {
                currentToken.append(c);
                i++;
            }
            else // No Match - Invalid Token!
            {
                i++;
            }

            if (currentToken.length() > 0)
            {
                tokens.add(currentToken.toString());    
                previousToken = currentToken.toString();    
            }
        }   
        return tokens;
    }