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With inspiration from other libraries
There is a newer version: 2.12.15
/**
 * Logback: the reliable, generic, fast and flexible logging framework.
 * Copyright (C) 1999-2015, QOS.ch. All rights reserved.
 *
 * This program and the accompanying materials are dual-licensed under
 * either the terms of the Eclipse Public License v1.0 as published by
 * the Eclipse Foundation
 *
 *   or (per the licensee's choosing)
 *
 * under the terms of the GNU Lesser General Public License version 2.1
 * as published by the Free Software Foundation.
 */
package ch.qos.logback.core.helpers;

import java.util.regex.Pattern;

/**
 * Utility class for transforming strings.
 * 
 * @author Ceki Gülcü
 * @author Michael A. McAngus
 */
public class Transform {
    private static final String CDATA_START = "";
    private static final String CDATA_PSEUDO_END = "]]>";
    private static final String CDATA_EMBEDED_END = CDATA_END + CDATA_PSEUDO_END + CDATA_START;
    private static final int CDATA_END_LEN = CDATA_END.length();
    private static final Pattern UNSAFE_XML_CHARS = Pattern.compile("[\u0000-\u0008\u000b\u000c\u000e-\u001f<>&'\"]");

    /**
     * This method takes a string which may contain HTML tags (ie, <b>,
     * <table>, etc) and replaces any '<','>' ... characters with
     * respective predefined entity references.
     * 
     * @param input
     *          The text to be converted.
     */
    public static String escapeTags(final String input) {
        if (input == null || input.length() == 0 || !UNSAFE_XML_CHARS.matcher(input).find()) {
            return input;
        }
        StringBuffer buf = new StringBuffer(input);
        return escapeTags(buf);
    }

    /**
     * This method takes a StringBuilder which may contain HTML tags (ie, <b>,
     * <table>, etc) and replaces any '<' and '>' characters with
     * respective predefined entity references.
     * @param buf StringBuffer to transform
     * @return
     */
    public static String escapeTags(final StringBuffer buf) {
        for (int i = 0; i < buf.length(); i++) {
            char ch = buf.charAt(i);
            switch (ch) {
            case '\t':
            case '\n':
            case '\r':
                // These characters are below '\u0020' but are allowed:
                break;
            case '&':
                buf.replace(i, i + 1, "&");
                break;
            case '<':
                buf.replace(i, i + 1, "<");
                break;
            case '>':
                buf.replace(i, i + 1, ">");
                break;
            case '"':
                buf.replace(i, i + 1, """);
                break;
            case '\'':
                buf.replace(i, i + 1, "'");
                break;
            default:
                if (ch < '\u0020') {
                    // These characters are not allowed,
                    // replace them with "Object replacement character":
                    buf.replace(i, i + 1, "\uFFFD");
                }
                break;
            }
        }
        return buf.toString();
    }

    /**
     * Ensures that embedded CDEnd strings (]]>) are handled properly within
     * message, NDC and throwable tag text.
     * 
     * @param output
     *          Writer. The initial CDSutart () of
     *          the CDATA section are the responsibility of the calling method.
     * 
     * @param str
     *          The String that is inserted into an existing CDATA Section.
     */
    public static void appendEscapingCDATA(StringBuilder output, String str) {
        if (str == null) {
            return;
        }

        int end = str.indexOf(CDATA_END);

        if (end < 0) {
            output.append(str);

            return;
        }

        int start = 0;

        while (end > -1) {
            output.append(str.substring(start, end));
            output.append(CDATA_EMBEDED_END);
            start = end + CDATA_END_LEN;

            if (start < str.length()) {
                end = str.indexOf(CDATA_END, start);
            } else {
                return;
            }
        }

        output.append(str.substring(start));
    }
}