javaregexstringsplitequation

What is the right regex for my String.split()

I am splitting equation string into string array like this:

String[] equation_array = (equation.split("(?<=[-+×÷)(])|(?=[-+×÷)(])"));

Now for test string: 

test = "4+(2×5)"

result is fine:

test_array = {"4", "+", "(", "2",...}

but for test string:

test2 = "(2×5)+5"

I got string array:

test2_array = {"", "(", "×",...}.

So, problem is why does it add an empty string before ( in array after splitting?

Solution

  • This is actually known behavior in Java regex.

    To avoid this empty result use this negative lookahead based regex:

    String[] equation_array = "(2×5)+5".split("(?!^)((?<=[-+×÷)(])|(?=[-+×÷)(]))");
//=> ["(", "2", "×", "5", ")", "+", "5"]

    What (?!^) means is to avoid splitting at line start.