The following code demonstrates the bug:

import java.util.regex.*;
public class T {
    public static void main(String[] args) throws Throwable {
        String input = "\ud801\udc37";
        Pattern p = Pattern.compile(".+");
        Matcher m = p.matcher(input);
        m.region(0, 1);
        if (m.find()) {
            System.out.println(": " + m.group(0) + " : " + m.group(0).length());
        }
    }
}

It is expected to print a single high half of the surrogate pair, and its length of 1.
In reality, it prints
: 𐐷 : 2
i.e. the result crosses the boundary of a set region.