正则表达式问题 ' " \

Question

代码如下：
[code="java"]

System.out.println("\'");
System.out.println("\'");
System.out.println("\\'");
System.out.println("\\"");
String a = "'\"";
System.out.println("1" + a.replace("'", "\'"));
System.out.println("2" + a.replace("'", "\'"));
System.out.println("3" + a.replace("'", "\\'"));
System.out.println("4" + a.replace("'", "\\'"));
System.out.println("5" + a.replaceAll("\"", "\\""));
System.out.println("6" + a.replaceAll("\"", "\\\""));

[/code]

运行结果：
[code="java"]

'
\'
\'
\"
1'"
2\'"
3\'"
4\'"
5'"
6'\"

[/code]

问题：
3中第二个参数字面值为\',结果输出为\';为啥5中第二个参数字面值为\",结果输出不是\"而是"？怎么想都想不明白啊。

Answer 1

1、 ' 和 \' --都输出--> ' 也就是写不写\都一样

2、
"3" + a.replace("'", "\\'")
3\\'\" ---> 3\'" 即\ -->\ \'--->' \"--->"

3、replaceAll是正则表达式替换
"5" + a.replaceAll("\"", "\\"")

即
[code="java"]public String replaceAll(String regex, String replacement) {
return Pattern.compile(regex).matcher(this).replaceAll(replacement);
}[/code]
---------->
[code="java"]public String replaceAll(String replacement) {
reset();
boolean result = find();
if (result) {
StringBuffer sb = new StringBuffer();
do {
appendReplacement(sb, replacement);
result = find();
} while (result);
appendTail(sb);
return sb.toString();
}
return text.toString();
}
[/code]
重点是
public Matcher appendReplacement(StringBuffer sb, String replacement) {

    // If no match, return error
    if (first < 0)
        throw new IllegalStateException("No match available");

    // Process substitution string to replace group references with groups
    int cursor = 0;
    StringBuilder result = new StringBuilder();

    while (cursor < replacement.length()) {
        char nextChar = replacement.charAt(cursor);
       [color=red] if (nextChar == '\\') {
            cursor++;
            nextChar = replacement.charAt(cursor);
            result.append(nextChar);[/color]
            cursor++;
        } else if (nextChar == '$') {
            // Skip past $
            cursor++;
            // A StringIndexOutOfBoundsException is thrown if
            // this "$" is the last character in replacement
            // string in current implementation, a IAE might be
            // more appropriate.
            nextChar = replacement.charAt(cursor);
            int refNum = -1;
            if (nextChar == '{') {
                cursor++;
                StringBuilder gsb = new StringBuilder();
                while (cursor < replacement.length()) {
                    nextChar = replacement.charAt(cursor);
                    if (ASCII.isLower(nextChar) ||
                        ASCII.isUpper(nextChar) ||
                        ASCII.isDigit(nextChar)) {
                        gsb.append(nextChar);
                        cursor++;
                    } else {
                        break;
                    }

此处红色部分 即如果匹配\ 那么跳过，则
5'\\" ---> 5'\"--得到-->5'"