java - 解释 Java 字符串

Question

有什么方法可以将诸如“hello\r\n\world”之类的字符串解释为一个字符串，其中\r\n 已转换为它们的实际文字值。

场景是用户键入正则表达式替换表达式并在 UI 中键入\r\n。该 get 已转义，但我想从中获取实际解释的字符串。

Answer 1

我并不直接熟悉处理此问题的“简单”方法(即我注意到是否有可以处理此问题的内置库)。但是这样做的一种方法是阅读关于转义序列的 JLS 规范并编写一个单 channel 解析器，它可以找到并评估每个转义序列。 (检查 JLS 3.10.6 Escape Sequences for Character and String Literals )。

现在我知道了一个事实，有一些你很快就会忘记处理的怪事，例如八进制转义很棘手，因为它允许 1,2 或 3 位数字，并且在每种情况下允许不同的当它是转义时的值，当它只是整数时的值。

以字符串“\431”为例，这是与字符“1”连接的局部转义符“\43”，因为八进制转义符的第一位数字是 4，因此不能是完整的三位八进制值在这种情况下，它只允许 [0-3] 作为第一个数字。

就在大约一年前，我正在为 1.3 规范的一个子集共同编写一个 Java 编译器，它确实有转义序列，下面我包含了我们处理转义的代码——你实际上应该能够接受这个按原样编写代码并包含在 Utility 类中(如果您觉得仁慈，可以加入功劳):

private String processCharEscapes(String strVal) {
    // Loop helpers
    char[]          chrArr = strVal.toCharArray();
    StringBuilder   strOut = new StringBuilder(strVal.length());
    String          strEsc = "";    // Escape sequence, string buffer
    Character       chrBuf = null;  // Dangling character buffer

    // Control flags
    boolean inEscape    = false;    // In escape?
    boolean cbOctal3    = true;     // Can be octal 3-digit

    // Parse characters
    for(char c : chrArr) {
        if (!inEscape) {
            // Listen for start of escape sequence
            if (c == '\\') {
                inEscape = true;    // Enter escape
                strEsc = "";        // Reset escape buffer
                chrBuf = null;      // Reset dangling character buffer
                cbOctal3 = true;    // Reset cbOctal3 flag
            } else {
                strOut.append(c);   // Save to output
            }
        } else {
            // Determine escape termination
            if (strEsc.length() == 0) { // First character
                if (c >= 48 && c <= 55) {   // c is a digit [0-7]
                    if (c > 51) {   // c is a digit [4-7]
                        cbOctal3 = false;
                    }
                    strEsc += c;    // Save to buffer
                } else {    // c is a character
                    // Single-character escapes (will terminate escape loop)
                    if (c == 'n') {
                        inEscape = false;
                        strOut.append('\n');
                    } else if(c == 't') {
                        inEscape = false;
                        strOut.append('\t');
                    } else if(c == 'b') {
                        inEscape = false;
                        strOut.append('\b');
                    } else if(c == 'r') {
                        inEscape = false;
                        strOut.append('\r');
                    } else if(c == 'f') {
                        inEscape = false;
                        strOut.append('\f');
                    } else if(c == '\\') {
                        inEscape = false;
                        strOut.append('\\');
                    } else if(c == '\'') {
                        inEscape = false;
                        strOut.append('\'');
                    } else if(c == '"') {
                        inEscape = false;
                        strOut.append('"');
                    } else {
                        // Saw illegal character, after escape character '\'
                        System.err.println(ErrorType.SYNTAX_ERROR, "Illegal character escape sequence, unrecognised escape: \\" + c);
                    }
                }
            } else if(strEsc.length() == 1) {   // Second character (possibly)
                if (c >= 48 && c <= 55) {   // c is a digit [0-7]
                    strEsc += c;    // Save to buffer
                    if (!cbOctal3) {    // Terminate since !cbOctal3
                        inEscape = false;
                    }
                } else {
                    inEscape = false;   // Terminate since c is not a digit
                    chrBuf = c;         // Save dangling character
                }
            } else if(strEsc.length() == 2) {   // Third character (possibly)
                if (cbOctal3 && c >= 48 && c <= 55) {
                    strEsc += c;        // Save to buffer
                } else {
                    chrBuf = c;         // Save dangling character
                }
                inEscape = false;       // Will always terminate after third character, no matter what
            }

            // Did escape sequence terminate, at character c?
            if (!inEscape && strEsc.length() > 0) {
                // strEsc is legal 1-3 digit octal char code, convert and add
                strOut.append((char)Integer.parseInt(strEsc, 8));

                if (chrBuf != null) {   // There was a dangling character
                    // Check for chained escape sequences (e.g. \10\10)
                    if (chrBuf == '\\') {
                        inEscape = true;    // Enter escape
                        strEsc = "";        // Reset escape buffer
                        chrBuf = null;      // Reset dangling character buffer
                        cbOctal3 = true;    // Reset cbOctal3 flag
                    } else {
                        strOut.append(chrBuf);
                    }
                }
            }
        }
    }

    // Check for EOL-terminated escape sequence (special case)
    if (inEscape) {
        // strEsc is legal 1-3 digit octal char code, convert and add
        strOut.append((char)Integer.parseInt(strEsc, 8));

        if (chrBuf != null) {   // There was a dangling character
            strOut.append(chrBuf);
        }
    }

    return strOut.toString();
}

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