Efficiently split large strings in Java

Question

I have a large string that should be split at a certain character, if it is not preceded by another certain character.

Would is the most efficient way to do this?

An example: Split this string at ':', but not at "?:":

part1:part2:https?:example.com:anotherstring

What I have tried so far:

1. Regex (?<!\?):. Very slow.

2. First getting the indices where to split the string and then split it. Only efficient if there are not many split characters in the string.

3. Iterating over the string character by character. Efficient if there are not many protect characters (e.g. '?').

Answer 1

I fear you would have to go through the string and check if a ":" is preceded by a "?"

int lastIndex=0;
for(int index=string.indexOf(":"); index >= 0; index=string.indexOf(":", lastIndex)){
    if(index == 0 || string.charAt(index-1) != '?'){
        String splitString = string.subString(lastIndex, index);
        // add splitString to list or array
        lastIndex = index+1;
    }
}
// add string.subString(lastIndex) to list or array

Answer 2

You will have to test this very carefully (since I didn't do that), but using a regular expression in the split() might produce the results you want:

public static void main(String[] args) {
    String s = "Start.Teststring.Teststring1?.Teststring2.?Teststring3.?.End";
    String[] result = s.split("(?<!\\?)\\.(?!\\.)");
    System.out.println(String.join("|", result));
}

Output:

Start|Teststring|Teststring1?.Teststring2|?Teststring3|?.End

Note:
This only considers your example about splitting by dot if the dot is not preceded by an interrogation mark.

I don't think you will get a much more performant solution than the regex...