特殊构造（非捕获） 
(?:X) ，作为非捕获组 
(?idmsux-idmsux)  Nothing，但是将匹配标志i d m s u x on - off 
(?idmsux-idmsux:X)   ，作为带有给定标志 i d m s u x on - off 
(?=X) ，通过零宽度的正 lookahead 
(?!X) ，通过零宽度的负 lookahead 
(?<=X) ，通过零宽度的正 lookbehind 
(?<!X) ，通过零宽度的负 lookbehind 
(?>X),作为独立的非捕获组

import java.util.regex.Matcher;
import java.util.regex.Pattern;

public class MainClass {
    public static void main(String[] args) {
        Pattern pattern = Pattern.compile("(?=a).{3}");
        Matcher matcher = pattern.matcher("444a66b");

        while (matcher.find()) {
            System.out.println(matcher.group());
        }
    }
}

output:
a66

import java.util.regex.Matcher;
import java.util.regex.Pattern;

public class MainClass {
    public static void main(String[] args) {
        Pattern pattern = Pattern.compile(".{3}(?=a)");
        Matcher matcher = pattern.matcher("444a66b");

        while (matcher.find()) {
            System.out.println(matcher.group());
        }
    }
}

output: 444