新手Java语法问题：字符转义相关疑问?-灵析社区

Java语法相关，什么情况下需要多打一个"\"? import java.util.Scanner; import java.util.regex.Pattern; public class Main { public static void main(String[] args) { String str; Scanner sc = new Scanner(System.in); str = sc.nextLine(); int index = str.indexOf('['); int lastIndex = str.lastIndexOf(']'); str = str.substring(index + 1, lastIndex); String s = "1"; System.out.println("str:" + str); if (Pattern.matches(str, s)) System.out.println("str与s匹配成功！"); else System.out.println("str与s匹配失败！"); String str1 = "\\d"; System.out.println("str1:" + str1); if (Pattern.matches(str1, s)) System.out.println("str1与s匹配成功！"); else System.out.println("str1与s匹配成功！"); System.out.println(str1.equals(str)); } }