java匹配豎線的錯誤警示

String s1 = "|";
// 輸出 |
System.out.println(s1);
String s2 = s1.replaceAll("|","1");
// 輸出 1|1
System.out.println(s2);

String s3 = "<mfenced separators=\"|\">";
String s4 = s3.replaceAll("<mfenced separators=\"|\">","2");
// 這個輸出 2|2
System.out.println(s4);

// 因此之後複雜的公式匹配 | ,就先將|替換掉,再進行匹配
String s5 = s1.replaceAll("\\|", "1");
// 輸出 1
System.out.println(s5);

String s6 = s3.replaceAll("<mfenced separators=\"\\|\">", "2");
// 輸出 <mfenced separators="|">
System.out.println(s3);

// 若是想將s3替換爲2,如何操做
String s7 = s3.replaceAll("\\|", "豎");
s7 = s7.replaceAll("<mfenced separators=\"豎\">","2");
// 輸出 2
System.out.println(s7);