Insert space after every one or two characters in string

Question

I need to write code to correct user input. The input is a mathematical expression in infix form. In order for my calculations to be correct, i need each operand spaced away from the operator.

Can you please help me write a method that will provide the following expected results.

Expected input: 13*21-5+33 Expected Output: 13 * 21 - 5 + 33

Research I have done: I could only find out how to add spaces between every character in the string and that will not work for calculating expressions with double digit values. Ie: 1 3 * 2 1 - 5 + 3 3. How to insert Space after every Character of an existing String in Java?

Thanks

Answer 1

This solution normalizes the whitespace around each operator.

return expr.replaceAll("(?<=\\d)\\s*([-+/*])\\s*(?=\\d)", " $1 ");

Answer 2

Look for digits using regex then surround them with spaces:

string.replaceAll("\\d+", " $0 ").trim(); // trim in case numbers are first/last