/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 * 
 *      http://www.apache.org/licenses/LICENSE-2.0
 * 
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.apache.commons.lang;

import java.io.IOException;
import java.io.StringWriter;
import java.io.Writer;

import org.apache.commons.lang.exception.NestableRuntimeException;

/**
 * <p>Escapes and unescapes <code>String</code>s for
 * Java, Java Script, HTML, XML, and SQL.</p>
 *
 * @author Apache Jakarta Turbine
 * @author Purple Technology
 * @author <a href="mailto:alex@purpletech.com">Alexander Day Chaffee</a>
 * @author Antony Riley
 * @author Helge Tesgaard
 * @author <a href="sean@boohai.com">Sean Brown</a>
 * @author <a href="mailto:ggregory@seagullsw.com">Gary Gregory</a>
 * @author Phil Steitz
 * @author Pete Gieser
 * @since 2.0
 * @version $Id$
 */
public class StringEscapeUtils {

    private static final char CSV_DELIMITER = ',';
    private static final char CSV_QUOTE = '"';
    private static final String CSV_QUOTE_STR = String.valueOf(CSV_QUOTE);
    private static final char[] CSV_SEARCH_CHARS = new char[] {CSV_DELIMITER, CSV_QUOTE, CharUtils.CR, CharUtils.LF};

    /**
     * <p><code>StringEscapeUtils</code> instances should NOT be constructed in
     * standard programming.</p>
     *
     * <p>Instead, the class should be used as:
     * <pre>StringEscapeUtils.escapeJava("foo");</pre></p>
     *
     * <p>This constructor is public to permit tools that require a JavaBean
     * instance to operate.</p>
     */
    public StringEscapeUtils() {
      super();
    }

    // Java and JavaScript
    //--------------------------------------------------------------------------
    /**
     * <p>Escapes the characters in a <code>String</code> using Java String rules.</p>
     *
     * <p>Deals correctly with quotes and control-chars (tab, backslash, cr, ff, etc.) </p>
     *
     * <p>So a tab becomes the characters <code>'\\'</code> and
     * <code>'t'</code>.</p>
     *
     * <p>The only difference between Java strings and JavaScript strings
     * is that in JavaScript, a single quote must be escaped.</p>
     *
     * <p>Example:
     * <pre>
     * input string: He didn't say, "Stop!"
     * output string: He didn't say, \"Stop!\"
     * </pre>
     * </p>
     *
     * @param str  String to escape values in, may be null
     * @return String with escaped values, <code>null</code> if null string input
     */
    public static String escapeJava(String str) {
        return escapeJavaStyleString(str, false);
    }

    /**
     * <p>Escapes the characters in a <code>String</code> using Java String rules to
     * a <code>Writer</code>.</p>
     * 
     * <p>A <code>null</code> string input has no effect.</p>
     * 
     * @see #escapeJava(java.lang.String)
     * @param out  Writer to write escaped string into
     * @param str  String to escape values in, may be null
     * @throws IllegalArgumentException if the Writer is <code>null</code>
     * @throws IOException if error occurs on underlying Writer
     */
    public static void escapeJava(Writer out, String str) throws IOException {
        escapeJavaStyleString(out, str, false);
    }

    /**
     * <p>Escapes the characters in a <code>String</code> using JavaScript String rules.</p>
     * <p>Escapes any values it finds into their JavaScript String form.
     * Deals correctly with quotes and control-chars (tab, backslash, cr, ff, etc.) </p>
     *
     * <p>So a tab becomes the characters <code>'\\'</code> and
     * <code>'t'</code>.</p>
     *
     * <p>The only difference between Java strings and JavaScript strings
     * is that in JavaScript, a single quote must be escaped.</p>
     *
     * <p>Example:
     * <pre>
     * input string: He didn't say, "Stop!"
     * output string: He didn\'t say, \"Stop!\"
     * </pre>
     * </p>
     *
     * @param str  String to escape values in, may be null
     * @return String with escaped values, <code>null</code> if null string input
     */
    public static String escapeJavaScript(String str) {
        return escapeJavaStyleString(str, true);
    }

    /**
     * <p>Escapes the characters in a <code>String</code> using JavaScript String rules
     * to a <code>Writer</code>.</p>
     * 
     * <p>A <code>null</code> string input has no effect.</p>
     * 
     * @see #escapeJavaScript(java.lang.String)
     * @param out  Writer to write escaped string into
     * @param str  String to escape values in, may be null
     * @throws IllegalArgumentException if the Writer is <code>null</code>
     * @throws IOException if error occurs on underlying Writer
     **/
    public static void escapeJavaScript(Writer out, String str) throws IOException {
        escapeJavaStyleString(out, str, true);
    }

    /**
     * <p>Worker method for the {@link #escapeJavaScript(String)} method.</p>
     * 
     * @param str String to escape values in, may be null
     * @param escapeSingleQuotes escapes single quotes if <code>true</code>
     * @return the escaped string
     */
    private static String escapeJavaStyleString(String str, boolean escapeSingleQuotes) {
        if (str == null) {
            return null;
        }
        try {
            StringWriter writer = new StringWriter(str.length() * 2);
            escapeJavaStyleString(writer, str, escapeSingleQuotes);
            return writer.toString();
        } catch (IOException ioe) {
            // this should never ever happen while writing to a StringWriter
            ioe.printStackTrace();
            return null;
        }
    }

    /**
     * <p>Worker method for the {@link #escapeJavaScript(String)} method.</p>
     * 
     * @param out write to receieve the escaped string
     * @param str String to escape values in, may be null
     * @param escapeSingleQuote escapes single quotes if <code>true</code>
     * @throws IOException if an IOException occurs
     */
    private static void escapeJavaStyleString(Writer out, String str, boolean escapeSingleQuote) throws IOException {
        if (out == null) {
            throw new IllegalArgumentException("The Writer must not be null");
        }
        if (str == null) {
            return;
        }
        int sz;
        sz = str.length();
        for (int i = 0; i < sz; i++) {
            char ch = str.charAt(i);

            // handle unicode
            if (ch > 0xfff) {
                out.write("\\u" + hex(ch));
            } else if (ch > 0xff) {
                out.write("\\u0" + hex(ch));
            } else if (ch > 0x7f) {
                out.write("\\u00" + hex(ch));
            } else if (ch < 32) {
                switch (ch) {
                    case '\b':
                        out.write('\\');
                        out.write('b');
                        break;
                    case '\n':
                        out.write('\\');
                        out.write('n');
                        break;
                    case '\t':
                        out.write('\\');
                        out.write('t');
                        break;
                    case '\f':
                        out.write('\\');
                        out.write('f');
                        break;
                    case '\r':
                        out.write('\\');
                        out.write('r');
                        break;
                    default :
                        if (ch > 0xf) {
                            out.write("\\u00" + hex(ch));
                        } else {
                            out.write("\\u000" + hex(ch));
                        }
                        break;
                }
            } else {
                switch (ch) {
                    case '\'':
                        if (escapeSingleQuote) {
                          out.write('\\');
                        }
                        out.write('\'');
                        break;
                    case '"':
                        out.write('\\');
                        out.write('"');
                        break;
                    case '\\':
                        out.write('\\');
                        out.write('\\');
                        break;
                    case '/':
                        out.write('\\');
                        out.write('/');
                        break;
                    default :
                        out.write(ch);
                        break;
                }
            }
        }
    }

    /**
     * <p>Returns an upper case hexadecimal <code>String</code> for the given
     * character.</p>
     * 
     * @param ch The character to convert.
     * @return An upper case hexadecimal <code>String</code>
     */
    private static String hex(char ch) {
        return Integer.toHexString(ch).toUpperCase();
    }

    /**
     * <p>Unescapes any Java literals found in the <code>String</code>.
     * For example, it will turn a sequence of <code>'\'</code> and
     * <code>'n'</code> into a newline character, unless the <code>'\'</code>
     * is preceded by another <code>'\'</code>.</p>
     * 
     * @param str  the <code>String</code> to unescape, may be null
     * @return a new unescaped <code>String</code>, <code>null</code> if null string input
     */
    public static String unescapeJava(String str) {
        if (str == null) {
            return null;
        }
        try {
            StringWriter writer = new StringWriter(str.length());
            unescapeJava(writer, str);
            return writer.toString();
        } catch (IOException ioe) {
            // this should never ever happen while writing to a StringWriter
            ioe.printStackTrace();
            return null;
        }
    }

    /**
     * <p>Unescapes any Java literals found in the <code>String</code> to a
     * <code>Writer</code>.</p>
     *
     * <p>For example, it will turn a sequence of <code>'\'</code> and
     * <code>'n'</code> into a newline character, unless the <code>'\'</code>
     * is preceded by another <code>'\'</code>.</p>
     * 
     * <p>A <code>null</code> string input has no effect.</p>
     * 
     * @param out  the <code>Writer</code> used to output unescaped characters
     * @param str  the <code>String</code> to unescape, may be null
     * @throws IllegalArgumentException if the Writer is <code>null</code>
     * @throws IOException if error occurs on underlying Writer
     */
    public static void unescapeJava(Writer out, String str) throws IOException {
        if (out == null) {
            throw new IllegalArgumentException("The Writer must not be null");
        }
        if (str == null) {
            return;
        }
        int sz = str.length();
        StringBuffer unicode = new StringBuffer(4);
        boolean hadSlash = false;
        boolean inUnicode = false;
        for (int i = 0; i < sz; i++) {
            char ch = str.charAt(i);
            if (inUnicode) {
                // if in unicode, then we're reading unicode
                // values in somehow
                unicode.append(ch);
                if (unicode.length() == 4) {
                    // unicode now contains the four hex digits
                    // which represents our unicode character
                    try {
                        int value = Integer.parseInt(unicode.toString(), 16);
                        out.write((char) value);
                        unicode.setLength(0);
                        inUnicode = false;
                        hadSlash = false;
                    } catch (NumberFormatException nfe) {
                        throw new NestableRuntimeException("Unable to parse unicode value: " + unicode, nfe);
                    }
                }
                continue;
            }
            if (hadSlash) {
                // handle an escaped value
                hadSlash = false;
                switch (ch) {
                    case '\\':
                        out.write('\\');
                        break;
                    case '\'':
                        out.write('\'');
                        break;
                    case '\"':
                        out.write('"');
                        break;
                    case 'r':
                        out.write('\r');
                        break;
                    case 'f':
                        out.write('\f');
                        break;
                    case 't':
                        out.write('\t');
                        break;
                    case 'n':
                        out.write('\n');
                        break;
                    case 'b':
                        out.write('\b');
                        break;
                    case 'u':
                        {
                            // uh-oh, we're in unicode country....
                            inUnicode = true;
                            break;
                        }
                    default :
                        out.write(ch);
                        break;
                }
                continue;
            } else if (ch == '\\') {
                hadSlash = true;
                continue;
            }
            out.write(ch);
        }
        if (hadSlash) {
            // then we're in the weird case of a \ at the end of the
            // string, let's output it anyway.
            out.write('\\');
        }
    }

    /**
     * <p>Unescapes any JavaScript literals found in the <code>String</code>.</p>
     *
     * <p>For example, it will turn a sequence of <code>'\'</code> and <code>'n'</code>
     * into a newline character, unless the <code>'\'</code> is preceded by another
     * <code>'\'</code>.</p>
     *
     * @see #unescapeJava(String)
     * @param str  the <code>String</code> to unescape, may be null
     * @return A new unescaped <code>String</code>, <code>null</code> if null string input
     */
    public static String unescapeJavaScript(String str) {
        return unescapeJava(str);
    }

    /**
     * <p>Unescapes any JavaScript literals found in the <code>String</code> to a
     * <code>Writer</code>.</p>
     *
     * <p>For example, it will turn a sequence of <code>'\'</code> and <code>'n'</code>
     * into a newline character, unless the <code>'\'</code> is preceded by another
     * <code>'\'</code>.</p>
     *
     * <p>A <code>null</code> string input has no effect.</p>
     * 
     * @see #unescapeJava(Writer,String)
     * @param out  the <code>Writer</code> used to output unescaped characters
     * @param str  the <code>String</code> to unescape, may be null
     * @throws IllegalArgumentException if the Writer is <code>null</code>
     * @throws IOException if error occurs on underlying Writer
     */
    public static void unescapeJavaScript(Writer out, String str) throws IOException {
        unescapeJava(out, str);
    }

    // HTML and XML
    //--------------------------------------------------------------------------
    /**
     * <p>Escapes the characters in a <code>String</code> using HTML entities.</p>
     *
     * <p>
     * For example:
     * </p> 
     * <p><code>"bread" & "butter"</code></p>
     * becomes:
     * <p>
     * <code>&amp;quot;bread&amp;quot; &amp;amp; &amp;quot;butter&amp;quot;</code>.
     * </p>
     *
     * <p>Supports all known HTML 4.0 entities, including funky accents.
     * Note that the commonly used apostrophe escape character (&amp;apos;)
     * is not a legal entity and so is not supported). </p>
     *
     * @param str  the <code>String</code> to escape, may be null
     * @return a new escaped <code>String</code>, <code>null</code> if null string input
     * 
     * @see #unescapeHtml(String)
     * @see <a href="http://hotwired.lycos.com/webmonkey/reference/special_characters/">ISO Entities</a>
     * @see <a href="http://www.w3.org/TR/REC-html32#latin1">HTML 3.2 Character Entities for ISO Latin-1</a>
     * @see <a href="http://www.w3.org/TR/REC-html40/sgml/entities.html">HTML 4.0 Character entity references</a>
     * @see <a href="http://www.w3.org/TR/html401/charset.html#h-5.3">HTML 4.01 Character References</a>
     * @see <a href="http://www.w3.org/TR/html401/charset.html#code-position">HTML 4.01 Code positions</a>
     */
    public static String escapeHtml(String str) {
        if (str == null) {
            return null;
        }
        try {
            StringWriter writer = new StringWriter ((int)(str.length() * 1.5));
            escapeHtml(writer, str);
            return writer.toString();
        } catch (IOException e) {
            //assert false;
            //should be impossible
            e.printStackTrace();
            return null;
        }
    }

    /**
     * <p>Escapes the characters in a <code>String</code> using HTML entities and writes
     * them to a <code>Writer</code>.</p>
     *
     * <p>
     * For example:
     * </p> 
     * <code>"bread" & "butter"</code>
     * <p>becomes:</p>
     * <code>&amp;quot;bread&amp;quot; &amp;amp; &amp;quot;butter&amp;quot;</code>.
     *
     * <p>Supports all known HTML 4.0 entities, including funky accents.
     * Note that the commonly used apostrophe escape character (&amp;apos;)
     * is not a legal entity and so is not supported). </p>
     *
     * @param writer  the writer receiving the escaped string, not null
     * @param string  the <code>String</code> to escape, may be null
     * @throws IllegalArgumentException if the writer is null
     * @throws IOException when <code>Writer</code> passed throws the exception from
     *                                       calls to the {@link Writer#write(int)} methods.
     * 
     * @see #escapeHtml(String)
     * @see #unescapeHtml(String)
     * @see <a href="http://hotwired.lycos.com/webmonkey/reference/special_characters/">ISO Entities</a>
     * @see <a href="http://www.w3.org/TR/REC-html32#latin1">HTML 3.2 Character Entities for ISO Latin-1</a>
     * @see <a href="http://www.w3.org/TR/REC-html40/sgml/entities.html">HTML 4.0 Character entity references</a>
     * @see <a href="http://www.w3.org/TR/html401/charset.html#h-5.3">HTML 4.01 Character References</a>
     * @see <a href="http://www.w3.org/TR/html401/charset.html#code-position">HTML 4.01 Code positions</a>
     */
    public static void escapeHtml(Writer writer, String string) throws IOException {
        if (writer == null ) {
            throw new IllegalArgumentException ("The Writer must not be null.");
        }
        if (string == null) {
            return;
        }
        Entities.HTML40.escape(writer, string);
    }

    //-----------------------------------------------------------------------
    /**
     * <p>Unescapes a string containing entity escapes to a string
     * containing the actual Unicode characters corresponding to the
     * escapes. Supports HTML 4.0 entities.</p>
     *
     * <p>For example, the string "&amp;lt;Fran&amp;ccedil;ais&amp;gt;"
     * will become "&lt;Fran&ccedil;ais&gt;"</p>
     *
     * <p>If an entity is unrecognized, it is left alone, and inserted
     * verbatim into the result string. e.g. "&amp;gt;&amp;zzzz;x" will
     * become "&gt;&amp;zzzz;x".</p>
     *
     * @param str  the <code>String</code> to unescape, may be null
     * @return a new unescaped <code>String</code>, <code>null</code> if null string input
     * @see #escapeHtml(Writer, String)
     */
    public static String unescapeHtml(String str) {
        if (str == null) {
            return null;
        }
        try {
            StringWriter writer = new StringWriter ((int)(str.length() * 1.5));
            unescapeHtml(writer, str);
            return writer.toString();
        } catch (IOException e) {
            //assert false;
            //should be impossible
            e.printStackTrace();
            return null;
        }
    }

    /**
     * <p>Unescapes a string containing entity escapes to a string
     * containing the actual Unicode characters corresponding to the
     * escapes. Supports HTML 4.0 entities.</p>
     *
     * <p>For example, the string "&amp;lt;Fran&amp;ccedil;ais&amp;gt;"
     * will become "&lt;Fran&ccedil;ais&gt;"</p>
     *
     * <p>If an entity is unrecognized, it is left alone, and inserted
     * verbatim into the result string. e.g. "&amp;gt;&amp;zzzz;x" will
     * become "&gt;&amp;zzzz;x".</p>
     *
     * @param writer  the writer receiving the unescaped string, not null
     * @param string  the <code>String</code> to unescape, may be null
     * @throws IllegalArgumentException if the writer is null
     * @throws IOException if an IOException occurs
     * @see #escapeHtml(String)
     */
    public static void unescapeHtml(Writer writer, String string) throws IOException {
        if (writer == null ) {
            throw new IllegalArgumentException ("The Writer must not be null.");
        }
        if (string == null) {
            return;
        }
        Entities.HTML40.unescape(writer, string);
    }

    //-----------------------------------------------------------------------
    /**
     * <p>Escapes the characters in a <code>String</code> using XML entities.</p>
     *
     * <p>For example: <tt>"bread" & "butter"</tt> =>
     * <tt>&amp;quot;bread&amp;quot; &amp;amp; &amp;quot;butter&amp;quot;</tt>.
     * </p>
     *
     * <p>Supports only the five basic XML entities (gt, lt, quot, amp, apos).
     * Does not support DTDs or external entities.</p>
     *
     * <p>Note that unicode characters greater than 0x7f are currently escaped to 
     *    their numerical \\u equivalent. This may change in future releases. </p>
     *
     * @param writer  the writer receiving the unescaped string, not null
     * @param str  the <code>String</code> to escape, may be null
     * @throws IllegalArgumentException if the writer is null
     * @throws IOException if there is a problem writing
     * @see #unescapeXml(java.lang.String)
     */
    public static void escapeXml(Writer writer, String str) throws IOException {
        if (writer == null ) {
            throw new IllegalArgumentException ("The Writer must not be null.");
        }
        if (str == null) {
            return;
        }
        Entities.XML.escape(writer, str);
    }

    /**
     * <p>Escapes the characters in a <code>String</code> using XML entities.</p>
     *
     * <p>For example: <tt>"bread" & "butter"</tt> =>
     * <tt>&amp;quot;bread&amp;quot; &amp;amp; &amp;quot;butter&amp;quot;</tt>.
     * </p>
     *
     * <p>Supports only the five basic XML entities (gt, lt, quot, amp, apos).
     * Does not support DTDs or external entities.</p>
     *
     * <p>Note that unicode characters greater than 0x7f are currently escaped to 
     *    their numerical \\u equivalent. This may change in future releases. </p>
     *
     * @param str  the <code>String</code> to escape, may be null
     * @return a new escaped <code>String</code>, <code>null</code> if null string input
     * @see #unescapeXml(java.lang.String)
     */
    public static String escapeXml(String str) {
        if (str == null) {
            return null;
        }
        return Entities.XML.escape(str);
    }

    //-----------------------------------------------------------------------
    /**
     * <p>Unescapes a string containing XML entity escapes to a string
     * containing the actual Unicode characters corresponding to the
     * escapes.</p>
     *
     * <p>Supports only the five basic XML entities (gt, lt, quot, amp, apos).
     * Does not support DTDs or external entities.</p>
     *
     * <p>Note that numerical \\u unicode codes are unescaped to their respective 
     *    unicode characters. This may change in future releases. </p>
     *
     * @param writer  the writer receiving the unescaped string, not null
     * @param str  the <code>String</code> to unescape, may be null
     * @throws IllegalArgumentException if the writer is null
     * @throws IOException if there is a problem writing
     * @see #escapeXml(String)
     */
    public static void unescapeXml(Writer writer, String str) throws IOException {
        if (writer == null ) {
            throw new IllegalArgumentException ("The Writer must not be null.");
        }
        if (str == null) {
            return;
        }
        Entities.XML.unescape(writer, str);
    }

    /**
     * <p>Unescapes a string containing XML entity escapes to a string
     * containing the actual Unicode characters corresponding to the
     * escapes.</p>
     *
     * <p>Supports only the five basic XML entities (gt, lt, quot, amp, apos).
     * Does not support DTDs or external entities.</p>
     *
     * <p>Note that numerical \\u unicode codes are unescaped to their respective 
     *    unicode characters. This may change in future releases. </p>
     *
     * @param str  the <code>String</code> to unescape, may be null
     * @return a new unescaped <code>String</code>, <code>null</code> if null string input
     * @see #escapeXml(String)
     */
    public static String unescapeXml(String str) {
        if (str == null) {
            return null;
        }
        return Entities.XML.unescape(str);
    }

    //-----------------------------------------------------------------------
    /**
     * <p>Escapes the characters in a <code>String</code> to be suitable to pass to
     * an SQL query.</p>
     *
     * <p>For example,
     * <pre>statement.executeQuery("SELECT * FROM MOVIES WHERE TITLE='" + 
     *   StringEscapeUtils.escapeSql("McHale's Navy") + 
     *   "'");</pre>
     * </p>
     *
     * <p>At present, this method only turns single-quotes into doubled single-quotes
     * (<code>"McHale's Navy"</code> => <code>"McHale''s Navy"</code>). It does not
     * handle the cases of percent (%) or underscore (_) for use in LIKE clauses.</p>
     *
     * see http://www.jguru.com/faq/view.jsp?EID=8881
     * @param str  the string to escape, may be null
     * @return a new String, escaped for SQL, <code>null</code> if null string input
     */
    public static String escapeSql(String str) {
        if (str == null) {
            return null;
        }
        return StringUtils.replace(str, "'", "''");
    }

    //-----------------------------------------------------------------------

    /**
     * <p>Returns a <code>String</code> value for a CSV column enclosed in double quotes,
     * if required.</p>
     *
     * <p>If the value contains a comma, newline or double quote, then the
     *    String value is returned enclosed in double quotes.</p>
     * </p>
     *
     * <p>Any double quote characters in the value are escaped with another double quote.</p>
     *
     * <p>If the value does not contain a comma, newline or double quote, then the
     *    String value is returned unchanged.</p>
     * </p>
     *
     * see <a href="http://en.wikipedia.org/wiki/Comma-separated_values">Wikipedia</a> and
     * <a href="http://tools.ietf.org/html/rfc4180">RFC 4180</a>.
     *
     * @param str the input CSV column String, may be null
     * @return the input String, enclosed in double quotes if the value contains a comma,
     * newline or double quote, <code>null</code> if null string input
     * @since 2.4
     */
    public static String escapeCsv(String str) {
        if (StringUtils.containsNone(str, CSV_SEARCH_CHARS)) {
            return str;
        }
        try {
            StringWriter writer = new StringWriter();
            escapeCsv(writer, str);
            return writer.toString();
        } catch (IOException ioe) {
            // this should never ever happen while writing to a StringWriter
            ioe.printStackTrace();
            return null;
        }
    }

    /**
     * <p>Writes a <code>String</code> value for a CSV column enclosed in double quotes,
     * if required.</p>
     *
     * <p>If the value contains a comma, newline or double quote, then the
     *    String value is written enclosed in double quotes.</p>
     * </p>
     *
     * <p>Any double quote characters in the value are escaped with another double quote.</p>
     *
     * <p>If the value does not contain a comma, newline or double quote, then the
     *    String value is written unchanged (null values are ignored).</p>
     * </p>
     *
     * see <a href="http://en.wikipedia.org/wiki/Comma-separated_values">Wikipedia</a> and
     * <a href="http://tools.ietf.org/html/rfc4180">RFC 4180</a>.
     *
     * @param str the input CSV column String, may be null
     * @param out Writer to write input string to, enclosed in double quotes if it contains
     * a comma, newline or double quote
     * @throws IOException if error occurs on underlying Writer
     * @since 2.4
     */
    public static void escapeCsv(Writer out, String str) throws IOException {
        if (StringUtils.containsNone(str, CSV_SEARCH_CHARS)) {
            if (str != null) {
                out.write(str);
            }
            return;
        }
        out.write(CSV_QUOTE);
        for (int i = 0; i < str.length(); i++) {
            char c = str.charAt(i);
            if (c == CSV_QUOTE) {
                out.write(CSV_QUOTE); // escape double quote
            }
            out.write(c);
        }
        out.write(CSV_QUOTE);
    }

    /**
     * <p>Returns a <code>String</code> value for an unescaped CSV column. </p>
     *
     * <p>If the value is enclosed in double quotes, and contains a comma, newline 
     *    or double quote, then quotes are removed. 
     * </p>
     *
     * <p>Any double quote escaped characters (a pair of double quotes) are unescaped 
     *    to just one double quote. </p>
     *
     * <p>If the value is not enclosed in double quotes, or is and does not contain a 
     *    comma, newline or double quote, then the String value is returned unchanged.</p>
     * </p>
     *
     * see <a href="http://en.wikipedia.org/wiki/Comma-separated_values">Wikipedia</a> and
     * <a href="http://tools.ietf.org/html/rfc4180">RFC 4180</a>.
     *
     * @param str the input CSV column String, may be null
     * @return the input String, with enclosing double quotes removed and embedded double 
     * quotes unescaped, <code>null</code> if null string input
     * @since 2.4
     */
    public static String unescapeCsv(String str) {
        if (str == null) {
            return null;
        }
        try {
            StringWriter writer = new StringWriter();
            unescapeCsv(writer, str);
            return writer.toString();
        } catch (IOException ioe) {
            // this should never ever happen while writing to a StringWriter
            ioe.printStackTrace();
            return null;
        }
    }

    /**
     * <p>Returns a <code>String</code> value for an unescaped CSV column. </p>
     *
     * <p>If the value is enclosed in double quotes, and contains a comma, newline 
     *    or double quote, then quotes are removed. 
     * </p>
     *
     * <p>Any double quote escaped characters (a pair of double quotes) are unescaped 
     *    to just one double quote. </p>
     *
     * <p>If the value is not enclosed in double quotes, or is and does not contain a 
     *    comma, newline or double quote, then the String value is returned unchanged.</p>
     * </p>
     *
     * see <a href="http://en.wikipedia.org/wiki/Comma-separated_values">Wikipedia</a> and
     * <a href="http://tools.ietf.org/html/rfc4180">RFC 4180</a>.
     *
     * @param str the input CSV column String, may be null
     * @param out Writer to write the input String to, with enclosing double quotes 
     * removed and embedded double quotes unescaped, <code>null</code> if null string input
     * @throws IOException if error occurs on underlying Writer
     * @since 2.4
     */
    public static void unescapeCsv(Writer out, String str) throws IOException {
        if (str == null) {
            return;
        }
        if (str.length() < 2) {
            out.write(str);
            return;
        }
        if ( str.charAt(0) != CSV_QUOTE || str.charAt(str.length() - 1) != CSV_QUOTE ) {
            out.write(str);
            return;
        }

        // strip quotes
        String quoteless = str.substring(1, str.length() - 1);

        if ( StringUtils.containsAny(quoteless, CSV_SEARCH_CHARS) ) {
            // deal with escaped quotes; ie) ""
            str = StringUtils.replace(quoteless, CSV_QUOTE_STR + CSV_QUOTE_STR, CSV_QUOTE_STR);
        }

        out.write(str);
    }

}