报错java.lang.IndexOutOfBoundsException: No group 8的原因和解决办法

出现场景

Java在调用String的replaceAll()方法时，报错Caused by: java.lang.IndexOutOfBoundsException: No group 8（数字可能也是1，2，3，4，5.。。。等等）；

出现该异常原因：索引越界，使用非法索引访问数组。 索引为负数或大于或等于数组的大小。取值的时候超出了数组的长度，比如数组长度是1，当你取第二个的时候就会报这个异常。

或者出现以下某一个报错的时候：
java.lang.IllegalArgumentException: character to be escaped is missing、Illegal group reference: group index is missing、named capturing group has 0 length name、named capturing group is missing trailing ‘}’、Illegal group reference等。

尝试运行以下Java代码 ：

情况1，出现报错java.lang.IndexOutOfBoundsException: No group

    @Test
    public void replaceTest(){
        String regex = "s";
        String replacement = "$122";
        String str = "test str replace.";
        String str1 = str.replaceAll(regex, replacement);
        System.out.println(str1);
    }

会出现以下报错:

情况2:出现报错：java.lang.IllegalArgumentException: Illegal group reference

@Test
   public void replaceTest(){
       String regex = "s";
       String replacement = "$f";
       String str = "test str replace.";
       String str1 = str.replaceAll(regex, replacement);
       System.out.println(str1);
   }

会出现以下报错：

原因分析：

String的replaceAll(regex, replacement)方法会调用java.util.regex.Matcher.appendReplacement(sb, replacement)方法

我们来看一下这个方法是的源码就知道问题所在了

 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);
            if (nextChar == '\\') {
                cursor++;
                if (cursor == replacement.length())
                    throw new IllegalArgumentException(
                        "character to be escaped is missing");
                nextChar = replacement.charAt(cursor);
                result.append(nextChar);
                cursor++;
            } else if (nextChar == '$') {
                // Skip past $
                cursor++;
                // Throw IAE if this "$" is the last character in replacement
                if (cursor == replacement.length())
                   throw new IllegalArgumentException(
                        "Illegal group reference: group index is missing");
                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;
                        }
                    }
                    if (gsb.length() == 0)
                        throw new IllegalArgumentException(
                            "named capturing group has 0 length name");
                    if (nextChar != '}')
                        throw new IllegalArgumentException(
                            "named capturing group is missing trailing '}'");
                    String gname = gsb.toString();
                    if (ASCII.isDigit(gname.charAt(0)))
                        throw new IllegalArgumentException(
                            "capturing group name {" + gname +
                            "} starts with digit character");
                    if (!parentPattern.namedGroups().containsKey(gname))
                        throw new IllegalArgumentException(
                            "No group with name {" + gname + "}");
                    refNum = parentPattern.namedGroups().get(gname);
                    cursor++;
                } else {
                    // The first number is always a group
                    refNum = (int)nextChar - '0';
                    if ((refNum < 0)||(refNum > 9))
                        throw new IllegalArgumentException(
                            "Illegal group reference");
                    cursor++;
                    // Capture the largest legal group string
                    boolean done = false;
                    while (!done) {
                        if (cursor >= replacement.length()) {
                            break;
                        }
                        int nextDigit = replacement.charAt(cursor) - '0';
                        if ((nextDigit < 0)||(nextDigit > 9)) { // not a number
                            break;
                        }
                        int newRefNum = (refNum * 10) + nextDigit;
                        if (groupCount() < newRefNum) {
                            done = true;
                        } else {
                            refNum = newRefNum;
                            cursor++;
                        }
                    }
                }
                // Append group
                if (start(refNum) != -1 && end(refNum) != -1)
                    result.append(text, start(refNum), end(refNum));
            } else {
                result.append(nextChar);
                cursor++;
            }
        }
        // Append the intervening text
        sb.append(text, lastAppendPosition, first);
        // Append the match substitution
        sb.append(result);

        lastAppendPosition = last;
        return this;
    }

可以从源码看出，appendReplacement()方法对反斜杠\和美元符号$
进行了特殊处理。上述示例的代码可以看出，需要替换的字符串中含有了$，所以appendReplacement()对其进行了特殊处理。
出现以上错误的原因是：String的replaceAll(regex, replacement)方法的第一个参数支持正则表达式，如果参数replacement中出现符号“$”,会按照$1$2的分组模式进行匹配。当编译器发现“$”后跟的不是整数的时候，就会抛出“Illegal group reference”的异常。当编译器发现$后面跟的是数字，却找不到分组时，就会抛出“No group ”的异常。

解决方法

需要将字符串中的反斜杠\\和美元符号$进行转义。可以使用JDK提供的方法，对特殊字符进行处理：

// 将replacement中的特殊字符进行转义
replacement = java.util.regex.Matcher.quoteReplacement(replacement);

前面的代码可以改成：

    @Test
    public void replaceTest(){
        String regex = "s";
        String replacement = "$f";
        String str = "test str replace.";
        replacement = java.util.regex.Matcher.quoteReplacement(replacement);
        String str1 = str.replaceAll(regex, replacement);
        System.out.println(str1);
    }