java正则匹配6-20字母或数字或字母、数字两者组合

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

public class AccountUtils {

    public static void main(String[] args) {

        String input="";
        String regex="^(\\w){6,20}$";
        Pattern compile = Pattern.compile(regex);
        Matcher matcher = compile.matcher(input);

        System.out.println(matcher.matches());
    }
}