Java Splitting With Math Expression

Question

I am trying to split a Math Expression.

String number = "100+500";

String[] split = new String[3];

I want to make

• split[0] = "100"
• split[1] = "+"
• split[2] = "500"

I tried this but I don't know what to write for splitting.

split = number.split(????);

Answer 1

You want to split between digits and non-digits without consuming any input... you need look arounds:

String[] split = number.split("(?<=\\d)(?=\\D)|(?<=\\D)(?=\\d)");

What the heck is that train wreck of a regex?

It's expressing the initial sentence of this answer:

• (?<=\d) means the previous character is a digit
• (?=\D) means the next character is a non-digit
• (?<=\d)(?=\D) together will match between a digit and a non-digit
• regexA|regexB means either regexA or regexB is matched, which is used as above points, but non-digit then digit for the visa-versa logic

An important point is that look arounds are non-consuming, so the split doesn't gobble up any of the input during the split.

---

Here's some test code:

String number = "100+500-123/456*789";
String[] split = number.split("(?<=\\d)(?=\\D)|(?<=\\D)(?=\\d)");
System.out.println(Arrays.toString(split));

Output:

[100, +, 500, -, 123, /, 456, *, 789]

---

To work with numbers that may have a decimal point, use this regex:

"(?<=[\\d.])(?=[^\\d.])|(?<=[^\\d.])(?=[\\d.])"

which effectively just add . to the characters that are a "number".

Answer 2

Off the bat, I don't know any library routine for the split. A custom splitting routine could be like this:

/**
 * Splits the given {@link String} at the operators +, -, * and /
 * 
 * @param string
 *            the {@link String} to be split.
 * @throws NullPointerException
 *             when the given {@link String} is null.
 * @return a {@link List} containing the split string and the operators.
 */
public List<String> split(String string) throws NullPointerException {
    if (string == null)
        throw new NullPointerException("the given string is null!");
    List<String> result = new ArrayList<String>();

    // operators to split upon
    String[] operators = new String[] { "+", "-", "*", "/" };

    int index = 0;
    while (index < string.length()) {
        // find the index of the nearest operator
        int minimum = string.length();
        for (String operator : operators) {
            int i = string.indexOf(operator, index);
            if (i > -1)
                minimum = Math.min(minimum, i);
        }

        // if an operator is found, split the string
        if (minimum < string.length()) {
            result.add(string.substring(index, minimum));
            result.add("" + string.charAt(minimum));
            index = minimum + 1;
        } else {
            result.add(string.substring(index));
            break;
        }
    }

    return result;
}

Some test code:

System.out.println(split("100+10*6+3"));
System.out.println(split("100+"));

Output:

[100, +, 10, *, 6, +, 3]
[100, +]