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?
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.
(?!^) works just as well. Recall that ^ itself is a zero-width assertion that matches a position that's not preceded by any character. It doesn't need to move backward to do is work, it only has to look at the preceding character (or lack of one). That means you can use this technique in JavaScript, too."(2×5)+5".split("(?!^)((?<=[-+×÷)(])|(?=[-+×÷)(]))"); will work just fine.You can add condition that not to split if before token is start of string like
"(?<=[-+×÷)(])|(?<!^)(?=[-+×÷)(])"
^^^^^^
What about looking backwards to make sure we're not at the start of the string, and looking forwards to make sure we're not at the end?
"(?<=[-+×÷)(])(?!$)|(?<!^)(?=[-+×÷)(])"
Here ^ and $ are start and end of string indicators and (?!...) and (?<!...) are negative lookahead and lookbehind.
problem is why does it add an empty string before
(in array after splitting?
Because for the input (2×5)+5 the regex used for splitting matches right at the start-of-string because of the positive look ahead (?=[-+×÷)(]).
(2×5)+5
↖
It matches right here before the (, resulting in an empty string: "".
My advice would be not to use regular expressions to parse mathematical expressions, there are more suitable algorithms for this.
