java.util.regex.PatternSyntaxException: Dangling meta character '+' near index 0 +

There are reserved character in Regex and you should scape these character to achieve what you want. For example, you can't use String.split("+"), you have to use String.split("\\+").

The correct operators would be:

String[] operators = new String[] {"-","\\+","/","\\*","x","\\^","X"};

in your case + * and ^ are treated with a special meaning, most often called as Metacharacters. String.split() method takes a regex expression as its argument and return a String array. To avoid treating above as a Metacharacters you need to use these escape sequences in your code "\\+" "\\*" "\\^"

modify your operator array like this

private String[] operators = new String[] {"-","\\+","/","\\*","x","\\^","X"};

for more detalis refere these links regex.Pattern and String.split()

you can use case String.valueOf('+');

Change: String[] split = nums.split(operator);

To this: String[] split = nums.split("\\" + operator);

edit: This will only work for standard operators, not the x or X. You'll have to change your String[] operators declaration actually, like the other answer mentioned. Personally though, I'd do some kind of input validation and do a replace() on x or X to be * instead