本人在專案中需要將資料庫中的\r\n轉換成html頁面可識別的<br />，於是使用了text.replaceAll("(\\r\\n|\\r|\\n|\\n\\r)", "<br />");來進行替換，發現竟然替換不了！！！列印輸出的內容中毫無變化。

然後嘗試換其他的方法Pattern替換等等，依舊無效。

後來偶然查看了下replaceAll 的原始碼，瞬間豁然開朗！程式碼如下:（解決方案在最後！！）

public String replaceAll(String regex, String replacement) {
    return Pattern.compile(regex).matcher(this).replaceAll(replacement);
}
 

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();
}
 

關鍵來了！！！

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++;
                nextChar = replacement.charAt(cursor);
                result.append(nextChar);
                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;
                        }
                    }
                    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;
    }
 

從上面這段程式碼中我們發現這樣一個片段

char nextChar = replacement.charAt(cursor);

if (nextChar == '\\') {
    cursor++;
    nextChar = replacement.charAt(cursor);
    result.append(nextChar);
    cursor++;
} 

也就是說它把連續的兩個反斜槓(‘\\’)變成了一個反斜槓(‘\’)

到這裡我想各位都已經明白了，咱們要替換的‘\\r’應該用‘\\\\r’來代替，用連續的4個反斜槓

也就是text.replaceAll("(\\\\r\\\\n|\\\\r|\\\\n|\\\\n\\\\r)", "<br />");

經過測試，這行程式碼完美的替換掉了內容中的換行字元。

這個問題坑了我快一個小時，放到這裡供各位參考。