Pattern p = Pattern.compile("cat");
Matcher m = p.matcher("one cat two cats in the yard");
StringBuffer sb = new StringBuffer();
while (m.find()) {m.appendReplacement(sb, "dog");
}
m.appendTail(sb);
System.out.println(sb.toString());

public Matcher appendReplacement(StringBuffer sb, String replacement) {// If no match, return errorif (first < 0) //first表示正则匹配到的第一个字符在字符串(one cat two cats in the yard)中的下标throw new IllegalStateException("No match available");// Process substitution string to replace group references with groupsint cursor = 0; //当前操作字符在替代字符串(dog)中的下标StringBuilder result = new StringBuilder();//解析替代字符串的结果//开始解析while (cursor < replacement.length()) {char nextChar = replacement.charAt(cursor);//获取当前操作的字符if (nextChar == '\\') {//如果当前操作的字符是转义符号//下面就是解析replacement转义符号的过程cursor++;//跳过转义符号if (cursor == replacement.length())throw new IllegalArgumentException("character to be escaped is missing");nextChar = replacement.charAt(cursor);//获取转义符号后面的字符result.append(nextChar);//添加到resultcursor++;//下标加一  回到上面while循环} else if (nextChar == '$') {//如果当前操作的字符是$//下面就是解析replacement中如何引用捕获组的内容// Skip past $cursor++;// 跳过$符号// Throw IAE if this "$" is the last character in replacementif (cursor == replacement.length())throw new IllegalArgumentException("Illegal group reference: group index is missing");nextChar = replacement.charAt(cursor);//获取$后面的字符int refNum = -1;if (nextChar == '{') {//解析命名捕获组cursor++;StringBuilder gsb = new StringBuilder();//提取${groupname}中的groupname字符串while (cursor < replacement.length()) {nextChar = replacement.charAt(cursor);//获取{的下一个字符if (ASCII.isLower(nextChar) ||ASCII.isUpper(nextChar) ||ASCII.isDigit(nextChar)) {//这个字符是大小写的字母或数字gsb.append(nextChar);cursor++;} else {//不是的话退出循环  后面判断是不是这个字符是不是}，不是就抛异常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();// group nameif (ASCII.isDigit(gname.charAt(0))) //group name 不能以数字开头throw new IllegalArgumentException("capturing group name {" + gname +"} starts with digit character");if (!parentPattern.namedGroups().containsKey(gname)) //捕获组的Map中要有相对应的group namethrow new IllegalArgumentException("No group with name {" + gname + "}");refNum = parentPattern.namedGroups().get(gname);//获取在捕获组中是第几个 begin with 1cursor++;} else {// The first number is always a grouprefNum = (int)nextChar - '0'; //减去字符0的ASCII码if ((refNum < 0)||(refNum > 9))//不是数字的话 抛异常throw new IllegalArgumentException("Illegal group reference");cursor++;// Capture the largest legal group stringboolean done = false;while (!done) {if (cursor >= replacement.length()) {//当前操作是最后一个字符或者后面已经没有字符了break;}int nextDigit = replacement.charAt(cursor) - '0';//减去字符0的ASCII码if ((nextDigit < 0)||(nextDigit > 9)) { // not a number  数字后面不是数字了break;}int newRefNum = (refNum * 10) + nextDigit;//还是数字  乘10累加上去if (groupCount() < newRefNum) {//累加后发现数值大于捕获组的总数done = true;//结束返回refNum中的值} else {//累加后的数值小于或等于捕获组总数 　继续循环下一个字符refNum = newRefNum;/cursor++;}}}// Append groupif (start(refNum) != -1 && end(refNum) != -1)result.append(text, start(refNum), end(refNum));   //获取对应捕获组的开始下标和结束下标} else {//正常字符直接添加result.append(nextChar);cursor++;}}// Append the intervening textsb.append(text, lastAppendPosition, first);//上一次添加位置到捕获组第一个字符位置// Append the match substitutionsb.append(result);//添加解析后的替代字符串lastAppendPosition = last;return this;
}