为什么Java中replaceAll替换\r\n失败？原因及解决方法详细说明

为什么Java中replaceAll替换rn失败？原因及解决方法详细说明并不只看表面做法，关键还要理解相关条件、限制和后续影响。

前言

本人在项目中需要将数据库中的rn转换成html页面可识别的<br />，于是使用了text.replaceAll("(rn|r|n|nr)", "<br />");来进行替换，发现竟然替换不了！！！打印输出的内容中毫无变化。（数据库文本----"这是一个段落rn"，发现rn变成了rn）

然后尝试换其他的方法Pattern替换等等，依旧无效。

后来偶然查看了下replaceAll 的源码

瞬间豁然开朗

代码如下：

（解决方案在最后！！）

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();}

关键来了！！！

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;    }

从上面这段代码中我们发现这样一个片段

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

也就是说它把连续的两个反斜杠(‘’)变成了一个反斜杠(‘’)

到这里我想各位都已经明白了，咱们要替换的‘r’应该用‘\r’来代替，用连续的4个反斜杠

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

这个问题主要出现在数据库读取出的数据中rn变成了rn，如果是正常的rn，只需要正常替换就可以了。

经过测试，这行代码完美的替换掉了内容中的换行字符。

这个问题坑了我快一个小时，放到这里供各位参考。

总结