diff --git a/core/src/main/java/org/jruby/ext/nkf/Command.java b/core/src/main/java/org/jruby/ext/nkf/Command.java
deleted file mode 100644
index adf38fab5609..000000000000
--- a/core/src/main/java/org/jruby/ext/nkf/Command.java
+++ /dev/null
@@ -1,59 +0,0 @@
-/***** BEGIN LICENSE BLOCK *****
- * Version: EPL 2.0/GPL 2.0/LGPL 2.1
- *
- * The contents of this file are subject to the Eclipse Public
- * 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.eclipse.org/legal/epl-v20.html
- *
- * Software distributed under the License is distributed on an "AS
- * IS" basis, WITHOUT WARRANTY OF ANY KIND, either express or
- * implied. See the License for the specific language governing
- * rights and limitations under the License.
- *
- * Copyright (C) 2011 Koichiro Ohba <koichiro@meadowy.org>
- *
- * Alternatively, the contents of this file may be used under the terms of
- * either of the GNU General Public License Version 2 or later (the "GPL"),
- * or the GNU Lesser General Public License Version 2.1 or later (the "LGPL"),
- * in which case the provisions of the GPL or the LGPL are applicable instead
- * of those above. If you wish to allow use of your version of this file only
- * under the terms of either the GPL or the LGPL, and not to allow others to
- * use your version of this file under the terms of the EPL, indicate your
- * decision by deleting the provisions above and replace them with the notice
- * and other provisions required by the GPL or the LGPL. If you do not delete
- * the provisions above, a recipient may use your version of this file under
- * the terms of any one of the EPL, the GPL or the LGPL.
- ***** END LICENSE BLOCK *****/
-
-package org.jruby.ext.nkf;
-
-import java.util.List;
-import java.util.ArrayList;
-
-public class Command {
-    private final List<Option> options = new ArrayList<Option>();
-    public boolean hasOption(String opt) {
-        for (Option option : options) {
-            if (opt.equals(option.getOpt())) return true;
-            if (opt.equals(option.getLongOpt())) return true;
-        }
-        return false;
-    }
-    public void addOption(Option opt) {
-        options.add(opt);
-    }
-    public Option getOption(String opt) {
-        for (Option option : options) {
-            if (opt.equals(option.getOpt())) return option;
-            if (opt.equals(option.getLongOpt())) return option;
-        }
-        return null;
-    }
-    public String getOptionValue(String opt) {
-        return getOption(opt).getValue();
-    }
-    public String toString() {
-        return options.toString();
-    }
-}
diff --git a/core/src/main/java/org/jruby/ext/nkf/CommandParser.java b/core/src/main/java/org/jruby/ext/nkf/CommandParser.java
deleted file mode 100644
index 834b41f9425f..000000000000
--- a/core/src/main/java/org/jruby/ext/nkf/CommandParser.java
+++ /dev/null
@@ -1,71 +0,0 @@
-/***** BEGIN LICENSE BLOCK *****
- * Version: EPL 2.0/GPL 2.0/LGPL 2.1
- *
- * The contents of this file are subject to the Eclipse Public
- * 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.eclipse.org/legal/epl-v20.html
- *
- * Software distributed under the License is distributed on an "AS
- * IS" basis, WITHOUT WARRANTY OF ANY KIND, either express or
- * implied. See the License for the specific language governing
- * rights and limitations under the License.
- *
- * Copyright (C) 2011 Koichiro Ohba <koichiro@meadowy.org>
- *
- * Alternatively, the contents of this file may be used under the terms of
- * either of the GNU General Public License Version 2 or later (the "GPL"),
- * or the GNU Lesser General Public License Version 2.1 or later (the "LGPL"),
- * in which case the provisions of the GPL or the LGPL are applicable instead
- * of those above. If you wish to allow use of your version of this file only
- * under the terms of either the GPL or the LGPL, and not to allow others to
- * use your version of this file under the terms of the EPL, indicate your
- * decision by deleting the provisions above and replace them with the notice
- * and other provisions required by the GPL or the LGPL. If you do not delete
- * the provisions above, a recipient may use your version of this file under
- * the terms of any one of the EPL, the GPL or the LGPL.
- ***** END LICENSE BLOCK *****/
-
-package org.jruby.ext.nkf;
-
-public class CommandParser {
-    public Command parse(Options opt, String args) {
-        Command cc = new Command();
-        String[] tokens = args.split("\\s");
-        for (int i = 0; i < tokens.length; i++) {
-            // long option
-            if (tokens[i].startsWith("--")) {
-                String s = stripDash(tokens[i]);
-                if (opt.hasLongOption(s)) {
-                    cc.addOption(opt.matchLongOption(s));
-                }
-            } else {
-                // short option
-                String s = stripDash(tokens[i]);
-                int max = s.length();
-                for (int j = 0; j < max; j++) {
-                    if (opt.hasShortOption(s)) {
-                        Option cmd = opt.matchShortOption(s);
-                        if (cmd.getValue() != null) {
-                            int op_len = cmd.getValue().length();
-                            s = s.substring(op_len);
-                            j = j + op_len;
-                        }
-                        cc.addOption(cmd);
-                    }
-                    s = s.substring(1);
-                }
-            }
-        }
-        return cc;
-    }
-    private String stripDash(String s) {
-        if (s.startsWith("--")) {
-            return s.substring(2, s.length());
-        } else if (s.startsWith("-")) {
-            return s.substring(1, s.length());
-        } else {
-            return s;
-        }
-    }
-}
diff --git a/core/src/main/java/org/jruby/ext/nkf/Option.java b/core/src/main/java/org/jruby/ext/nkf/Option.java
deleted file mode 100644
index c4949e9e77bc..000000000000
--- a/core/src/main/java/org/jruby/ext/nkf/Option.java
+++ /dev/null
@@ -1,81 +0,0 @@
-/***** BEGIN LICENSE BLOCK *****
- * Version: EPL 2.0/GPL 2.0/LGPL 2.1
- *
- * The contents of this file are subject to the Eclipse Public
- * 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.eclipse.org/legal/epl-v20.html
- *
- * Software distributed under the License is distributed on an "AS
- * IS" basis, WITHOUT WARRANTY OF ANY KIND, either express or
- * implied. See the License for the specific language governing
- * rights and limitations under the License.
- *
- * Copyright (C) 2011 Koichiro Ohba <koichiro@meadowy.org>
- *
- * Alternatively, the contents of this file may be used under the terms of
- * either of the GNU General Public License Version 2 or later (the "GPL"),
- * or the GNU Lesser General Public License Version 2.1 or later (the "LGPL"),
- * in which case the provisions of the GPL or the LGPL are applicable instead
- * of those above. If you wish to allow use of your version of this file only
- * under the terms of either the GPL or the LGPL, and not to allow others to
- * use your version of this file under the terms of the EPL, indicate your
- * decision by deleting the provisions above and replace them with the notice
- * and other provisions required by the GPL or the LGPL. If you do not delete
- * the provisions above, a recipient may use your version of this file under
- * the terms of any one of the EPL, the GPL or the LGPL.
- ***** END LICENSE BLOCK *****/
-
-package org.jruby.ext.nkf;
-
-import java.util.regex.Pattern;
-
-public class Option {
-    private final String opt;
-    private final String longOpt;
-    private boolean hasArg = false;
-    private String value = null;
-    private Pattern pattern;
-
-    public Option(String opt, String longOpt, String pattern) {
-        this.opt = opt;
-        this.longOpt = longOpt;
-        if (pattern != null) {
-            this.hasArg = true;
-            this.pattern = Pattern.compile(pattern);
-        }
-    }
-    String getOpt() { return opt; }
-    String getLongOpt() { return longOpt; }
-    boolean hasShortOpt() {
-        return opt != null;
-    }
-    boolean hasLongOpt() {
-        return longOpt != null;
-    }
-    boolean hasArg() {
-        return hasArg;
-    }
-    public String getValue() {
-        return value;
-    }
-    void setValue(String v) {
-        value = v;
-    }
-    String getKey() {
-        if (opt == null)
-            return longOpt;
-        else
-            return opt;
-    }
-    Pattern pattern() {
-        return pattern;
-    }
-    public String toString() {
-        return "[opt: " + opt
-            + " longOpt: " + longOpt
-            + " hasArg: " + hasArg
-            + " pattern: " + pattern
-            + " value: " + value + "]";
-    }
-}
diff --git a/core/src/main/java/org/jruby/ext/nkf/Options.java b/core/src/main/java/org/jruby/ext/nkf/Options.java
deleted file mode 100644
index 9687bf5b741d..000000000000
--- a/core/src/main/java/org/jruby/ext/nkf/Options.java
+++ /dev/null
@@ -1,110 +0,0 @@
-/***** BEGIN LICENSE BLOCK *****
- * Version: EPL 2.0/GPL 2.0/LGPL 2.1
- *
- * The contents of this file are subject to the Eclipse Public
- * 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.eclipse.org/legal/epl-v20.html
- *
- * Software distributed under the License is distributed on an "AS
- * IS" basis, WITHOUT WARRANTY OF ANY KIND, either express or
- * implied. See the License for the specific language governing
- * rights and limitations under the License.
- *
- * Copyright (C) 2011 Koichiro Ohba <koichiro@meadowy.org>
- *
- * Alternatively, the contents of this file may be used under the terms of
- * either of the GNU General Public License Version 2 or later (the "GPL"),
- * or the GNU Lesser General Public License Version 2.1 or later (the "LGPL"),
- * in which case the provisions of the GPL or the LGPL are applicable instead
- * of those above. If you wish to allow use of your version of this file only
- * under the terms of either the GPL or the LGPL, and not to allow others to
- * use your version of this file under the terms of the EPL, indicate your
- * decision by deleting the provisions above and replace them with the notice
- * and other provisions required by the GPL or the LGPL. If you do not delete
- * the provisions above, a recipient may use your version of this file under
- * the terms of any one of the EPL, the GPL or the LGPL.
- ***** END LICENSE BLOCK *****/
-
-package org.jruby.ext.nkf;
-
-import java.util.Map;
-import java.util.LinkedHashMap;
-import java.util.regex.Matcher;
-
-public class Options {
-    private final Map<String, Option> shortOpts = new LinkedHashMap<String, Option>();
-    private final Map<String, Option> longOpts = new LinkedHashMap<String, Option>();
-
-    public Options addOption(String opt) {
-        return addOption(opt, null);
-    }
-    public Options addOption(String opt, String longOpt) {
-        return addOption(opt, longOpt, null);
-    }
-    public Options addOption(String opt, String longOpt, String pattern) {
-        return addOption(new Option(opt, longOpt, pattern));
-    }
-    public Options addOption(Option opt) {
-        if (opt.hasLongOpt()) {
-            longOpts.put(opt.getLongOpt(), opt);
-        }
-        if (opt.hasShortOpt()) {
-            shortOpts.put(opt.getOpt(), opt);
-        }
-        return this;
-    }
-    boolean hasShortOption(String opt) {
-        for (Map.Entry<String , Option> e : shortOpts.entrySet()) {
-            if (opt.startsWith(e.getKey())) {
-                return true;
-            }
-        }
-        return false;
-    }
-    public Option matchShortOption(String opt) {
-        // independent of opt length
-        for (Map.Entry<String , Option> e : shortOpts.entrySet()) {
-            //System.out.println(opt + " = " + e.getKey());
-            if (opt.startsWith(e.getKey())) {
-                //System.out.println("match[" + e.getKey() + "]");
-                Option cmd = e.getValue();
-                if (cmd.hasArg()) {
-                    Matcher m = cmd.pattern().matcher(opt);
-                    if (m.find()) {
-                        //System.out.println("regix[" + m.group() + "]");
-                        cmd.setValue(m.group());
-                    }
-                }
-                return cmd;
-            }
-        }
-        return null;
-    }
-    boolean hasLongOption(String opt) {
-        for (Map.Entry<String , Option> e : longOpts.entrySet()) {
-            if (opt.startsWith(e.getKey())) {
-                return true;
-            }
-        }
-        return false;
-    }
-    Option matchLongOption(String opt) {
-        for (Map.Entry<String , Option> e : longOpts.entrySet()) {
-            //System.out.println(opt + " = " + e.getKey());
-            if (opt.startsWith(e.getKey())) {
-                //System.out.println("match[" + e.getKey() + "]");
-                Option cmd = e.getValue();
-                if (cmd.hasArg()) {
-                    Matcher m = cmd.pattern().matcher(opt);
-                    if (m.find()) {
-                        //System.out.println("regix[" + m.group() + "]");
-                        cmd.setValue(m.group(1));
-                    }
-                }
-                return cmd;
-            }
-        }
-        return null;
-    }
-}
diff --git a/core/src/main/java/org/jruby/ext/nkf/RubyNKF.java b/core/src/main/java/org/jruby/ext/nkf/RubyNKF.java
deleted file mode 100644
index 9875c309ba31..000000000000
--- a/core/src/main/java/org/jruby/ext/nkf/RubyNKF.java
+++ /dev/null
@@ -1,602 +0,0 @@
-/***** BEGIN LICENSE BLOCK *****
- * Version: EPL 2.0/GPL 2.0/LGPL 2.1
- *
- * The contents of this file are subject to the Eclipse Public
- * 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.eclipse.org/legal/epl-v20.html
- *
- * Software distributed under the License is distributed on an "AS
- * IS" basis, WITHOUT WARRANTY OF ANY KIND, either express or
- * implied. See the License for the specific language governing
- * rights and limitations under the License.
- *
- * Copyright (C) 2007-2011 Koichiro Ohba <koichiro@meadowy.org>
- *
- * Alternatively, the contents of this file may be used under the terms of
- * either of the GNU General Public License Version 2 or later (the "GPL"),
- * or the GNU Lesser General Public License Version 2.1 or later (the "LGPL"),
- * in which case the provisions of the GPL or the LGPL are applicable instead
- * of those above. If you wish to allow use of your version of this file only
- * under the terms of either the GPL or the LGPL, and not to allow others to
- * use your version of this file under the terms of the EPL, indicate your
- * decision by deleting the provisions above and replace them with the notice
- * and other provisions required by the GPL or the LGPL. If you do not delete
- * the provisions above, a recipient may use your version of this file under
- * the terms of any one of the EPL, the GPL or the LGPL.
- ***** END LICENSE BLOCK *****/
-
-package org.jruby.ext.nkf;
-
-import java.nio.ByteBuffer;
-import java.nio.CharBuffer;
-import java.nio.charset.CharacterCodingException;
-import java.nio.charset.Charset;
-import java.nio.charset.CharsetDecoder;
-import java.nio.charset.CharsetEncoder;
-import java.nio.charset.UnsupportedCharsetException;
-import java.util.ArrayList;
-import java.util.Map;
-import java.util.HashMap;
-
-import org.jcodings.Encoding;
-import org.jcodings.specific.ASCIIEncoding;
-import org.jcodings.specific.UTF8Encoding;
-import org.jcodings.transcode.EConv;
-import org.jcodings.transcode.EConvFlags;
-import org.jruby.Ruby;
-import org.jruby.RubyArray;
-import org.jruby.RubyModule;
-import org.jruby.RubyString;
-
-import org.jruby.anno.JRubyMethod;
-import org.jruby.anno.JRubyModule;
-import org.jruby.runtime.Helpers;
-import org.jruby.runtime.ThreadContext;
-import org.jruby.runtime.builtin.IRubyObject;
-import org.jruby.util.ByteList;
-import org.jruby.util.KCode;
-import org.jruby.util.Pack;
-import org.jruby.util.io.EncodingUtils;
-
-@JRubyModule(name="NKF")
-public class RubyNKF {
-    public static enum NKFCharset {
-        AUTO(0, "x-JISAutoDetect"),
-        // no ISO-2022-JP in jcodings
-        JIS(1, "ISO-2022-JP"),
-        EUC(2, "EUC-JP"),
-        SJIS(3, "Shift_JIS"),
-        BINARY(4, null),
-        NOCONV(4, null),
-        UNKNOWN(0, null),
-        ASCII(5, "iso-8859-1"),
-        UTF8(6, "UTF-8"),
-        UTF16(8, "UTF-16"),
-        UTF32(12, "UTF-32"),
-        OTHER(16, null),
-        BASE64(20, "base64"),
-        QENCODE(21, "qencode"),
-        MIME_DETECT(22, "MimeAutoDetect");
-
-        private NKFCharset(int value, String charset) {
-            this.value = value;
-            this.charset = charset;
-        }
-
-        public int getValue() {
-            return value;
-        }
-
-        public String getCharset() {
-            return charset;
-        }
-
-        private final int value;
-        private final String charset;
-    }
-
-    private static final ByteList BEGIN_MIME_STRING = new ByteList(ByteList.plain("=?"));
-    private static final ByteList END_MIME_STRING = new ByteList(ByteList.plain("?="));
-    private static final ByteList PACK_BASE64 = new ByteList(ByteList.plain("m"));
-    private static final ByteList PACK_QENCODE = new ByteList(ByteList.plain("M"));
-
-    public static final Map<Integer, String> NKFCharsetMap = new HashMap<Integer, String>(20, 1);
-
-    public static void load(Ruby runtime) {
-        createNKF(runtime);
-    }
-
-    public static void createNKF(Ruby runtime) {
-        final RubyModule NKF = runtime.defineModule("NKF");
-        final String version = "2.1.2";
-        final String relDate = "2011-09-08";
-
-        NKF.defineConstant("NKF_VERSION", runtime.newString(version));
-        NKF.defineConstant("NKF_RELEASE_DATE", runtime.newString(relDate));
-        NKF.defineConstant("VERSION", runtime.newString(version + ' ' + '(' + "JRuby" + '_' + relDate + ')'));
-
-        for ( NKFCharset charset : NKFCharset.values() ) {
-            NKFCharsetMap.put(charset.value, charset.name());
-
-            if (charset.value > 12 ) continue;
-            NKF.defineConstant(charset.name(), charsetMappedValue(runtime, charset));
-        }
-
-        NKF.defineAnnotatedMethods(RubyNKF.class);
-    }
-
-    @JRubyMethod(name = "guess", module = true)
-    public static IRubyObject guess(ThreadContext context, IRubyObject recv, IRubyObject s) {
-        return charsetMappedValue(context.runtime, guess(context, s));
-    }
-
-    public static NKFCharset guess(ThreadContext context, IRubyObject s) {
-        // TODO: Fix charset usage for JRUBY-4553
-        Ruby runtime = context.runtime;
-        if (!s.respondsTo("to_str")) {
-            throw runtime.newTypeError("can't convert " + s.getMetaClass() + " into String");
-        }
-        ByteList bytes = s.convertToString().getByteList();
-        ByteBuffer buf = ByteBuffer.wrap(bytes.getUnsafeBytes(), bytes.begin(), bytes.length());
-        CharsetDecoder decoder;
-        try {
-            decoder = Charset.forName("x-JISAutoDetect").newDecoder();
-        } catch (UnsupportedCharsetException e) {
-            throw runtime.newStandardError("charsets.jar is required to use NKF#guess. Please install JRE which supports m17n.");
-        }
-        try {
-            decoder.decode(buf);
-
-            if ( ! decoder.isCharsetDetected() ) {
-                return NKFCharset.UNKNOWN;
-            }
-            Charset charset = decoder.detectedCharset();
-            String name = charset.name();
-            if ("Shift_JIS".equals(name)) {
-                return NKFCharset.SJIS;
-            }
-            if ("Windows-31j".equalsIgnoreCase(name)) {
-                return NKFCharset.JIS;
-            }
-            if ("EUC-JP".equals(name)) {
-                return NKFCharset.EUC;
-            }
-            if ("ISO-2022-JP".equals(name)) {
-                return NKFCharset.JIS;
-            }
-        }
-        catch (CharacterCodingException e) {
-            // fall through and try direct encoding
-        }
-
-        if (bytes.getEncoding() == UTF8Encoding.INSTANCE) {
-            return NKFCharset.UTF8;
-        }
-        if (bytes.getEncoding().toString().startsWith("UTF-16")) {
-            return NKFCharset.UTF16;
-        }
-        if (bytes.getEncoding().toString().startsWith("UTF-32")) {
-            return NKFCharset.UTF32;
-        }
-        return NKFCharset.UNKNOWN;
-    }
-
-    private static IRubyObject charsetMappedValue(final Ruby runtime, final NKFCharset charset) {
-        final Encoding encoding;
-        switch (charset) {
-            case AUTO: case NOCONV: case UNKNOWN: return runtime.getNil();
-            case BINARY:
-                encoding = runtime.getEncodingService().getAscii8bitEncoding();
-                return runtime.getEncodingService().convertEncodingToRubyEncoding(encoding);
-        }
-
-        encoding = runtime.getEncodingService().getEncodingFromString(charset.getCharset());
-        return runtime.getEncodingService().convertEncodingToRubyEncoding(encoding);
-    }
-
-    @JRubyMethod(name = "guess1", module = true)
-    public static IRubyObject guess1(ThreadContext context, IRubyObject recv, IRubyObject str) {
-        return guess(context, recv, str);
-    }
-
-    @JRubyMethod(name = "guess2", module = true)
-    public static IRubyObject guess2(ThreadContext context, IRubyObject recv, IRubyObject str) {
-        return guess(context, recv, str);
-    }
-
-    @JRubyMethod(name = "nkf", module = true)
-    public static IRubyObject nkf(ThreadContext context, IRubyObject recv, IRubyObject opt, IRubyObject str) {
-        Ruby runtime = context.runtime;
-
-        if (!opt.respondsTo("to_str")) {
-            throw runtime.newTypeError("can't convert " + opt.getMetaClass() + " into String");
-        }
-
-        if (!str.respondsTo("to_str")) {
-            throw runtime.newTypeError("can't convert " + str.getMetaClass() + " into String");
-        }
-
-        Map<String, NKFCharset> options = parseOpt(opt.convertToString().toString());
-
-        if (options.get("input").getValue() == NKFCharset.AUTO.getValue()) {
-            options.put("input", guess(context, str));
-        }
-
-        ByteList bstr = str.convertToString().getByteList();
-        final Converter converter;
-        if (Converter.isMimeText(bstr, options)) {
-            converter = new MimeConverter(context, options);
-        } else {
-            converter = new DefaultConverter(context, options);
-        }
-
-        RubyString result = converter.convert(bstr);
-
-        if (options.get("mime-encode") == NKFCharset.BASE64) {
-            result = Converter.encodeMimeString(runtime, result, PACK_BASE64);
-        } else if (options.get("mime-encode") == NKFCharset.QENCODE) {
-            result = Converter.encodeMimeString(runtime, result, PACK_QENCODE);
-        }
-
-        return result;
-    }
-
-    public static Command parseOption(String s) {
-        Options options = new Options();
-        options.addOption("b");
-        options.addOption("u");
-        options.addOption("j", "jis");
-        options.addOption("s", "sjis");
-        options.addOption("e", "euc");
-        options.addOption("w", null, "[0-9][0-9]");
-        options.addOption("J", "jis-input");
-        options.addOption("S", "sjis-input");
-        options.addOption("E", "euc-input");
-        options.addOption("W", null, "[0-9][0-9]");
-        options.addOption("t");
-        options.addOption("i_");
-        options.addOption("o_");
-        options.addOption("r");
-        options.addOption("h1", "hiragana");
-        options.addOption("h2", "katakana");
-        options.addOption("h3", "katakana-hiragana");
-        options.addOption("T");
-        options.addOption("l");
-        options.addOption("f", null, "[0-9]+-[0-9]*");
-        options.addOption("F");
-        options.addOption("Z", null, "[0-3]");
-        options.addOption("X");
-        options.addOption("x");
-        options.addOption("B", null, "[0-2]");
-        options.addOption("I");
-        options.addOption("L", null, "[uwm]");
-        options.addOption("d");
-        options.addOption("c");
-        options.addOption("m", null, "[BQN0]");
-        options.addOption("M", null, "[BQ]");
-        options.addOption(null, "fj");
-        options.addOption(null, "unix");
-        options.addOption(null, "mac");
-        options.addOption(null, "msdos");
-        options.addOption(null, "windows");
-        options.addOption(null, "mime");
-        options.addOption(null, "base64");
-        options.addOption(null, "mime-input");
-        options.addOption(null, "base64-input");
-        options.addOption(null, "ic", "ic=(.*)");
-        options.addOption(null, "oc", "oc=(.*)");
-        options.addOption(null, "fb-skip");
-        options.addOption(null, "fb-html");
-        options.addOption(null, "fb-xml");
-        options.addOption(null, "fb-perl");
-        options.addOption(null, "fb-java");
-        options.addOption(null, "fb-subchar", "fb-subchar=(.*)");
-        options.addOption(null, "no-cp932ext");
-        options.addOption(null, "cap-input");
-        options.addOption(null, "url-input");
-        options.addOption(null, "numchar-input");
-        options.addOption(null, "no-best-fit-chars");
-
-        CommandParser parser = new CommandParser();
-        Command cmd = parser.parse(options, s);
-        return cmd;
-    }
-
-    private static Map<String, NKFCharset> parseOpt(String s) {
-        Map<String, NKFCharset> options = new HashMap<String, NKFCharset>();
-
-        // default options
-        options.put("input", NKFCharset.AUTO);
-        options.put("output", NKFCharset.JIS);
-        options.put("mime-decode", NKFCharset.MIME_DETECT);
-        options.put("mime-encode", NKFCharset.NOCONV);
-
-        Command cmd = parseOption(s);
-        if (cmd.hasOption("j")) {
-            options.put("output", NKFCharset.JIS);
-        }
-        if (cmd.hasOption("s")) {
-            options.put("output", NKFCharset.SJIS);
-        }
-        if (cmd.hasOption("e")) {
-            options.put("output", NKFCharset.EUC);
-        }
-        if (cmd.hasOption("w")) {
-            Option opt = cmd.getOption("w");
-            if ("32".equals(opt.getValue())) {
-                options.put("output", NKFCharset.UTF32);
-            } else if("16".equals(opt.getValue())) {
-                options.put("output", NKFCharset.UTF16);
-            } else {
-                options.put("output", NKFCharset.UTF8);
-            }
-        }
-        if (cmd.hasOption("J")) {
-            options.put("input", NKFCharset.JIS);
-        }
-        if (cmd.hasOption("S")) {
-            options.put("input", NKFCharset.SJIS);
-        }
-        if (cmd.hasOption("E")) {
-            options.put("input", NKFCharset.EUC);
-        }
-        if (cmd.hasOption("W")) {
-            Option opt = cmd.getOption("W");
-            if ("32".equals(opt.getValue())) {
-                options.put("input", NKFCharset.UTF32);
-            } else if("16".equals(opt.getValue())) {
-                options.put("input", NKFCharset.UTF16);
-            } else {
-                options.put("input", NKFCharset.UTF8);
-            }
-        }
-        if (cmd.hasOption("m")) {
-            Option opt = cmd.getOption("m");
-            if (opt.getValue() == null) {
-                options.put("mime-decode", NKFCharset.MIME_DETECT);
-            } else if ("B".equals(opt.getValue())) {
-                options.put("mime-decode", NKFCharset.BASE64);
-            } else if ("Q".equals(opt.getValue())) {
-                options.put("mime-decode", NKFCharset.QENCODE);
-            } else if ("N".equals(opt.getValue())) {
-                // TODO: non-strict option
-            } else if ("0".equals(opt.getValue())) {
-                options.put("mime-decode", NKFCharset.NOCONV);
-            }
-        }
-        if (cmd.hasOption("M")) {
-            Option opt = cmd.getOption("M");
-            if (opt.getValue() == null) {
-                options.put("mime-encode", NKFCharset.NOCONV);
-            } else if ("B".equals(opt.getValue())) {
-                options.put("mime-encode", NKFCharset.BASE64);
-            } else if ("Q".equals(opt.getValue())) {
-                options.put("mime-encode", NKFCharset.QENCODE);
-            }
-        }
-        if (cmd.hasOption("base64")) {
-            options.put("mime-encode", NKFCharset.BASE64);
-        }
-        if (cmd.hasOption("oc")) {
-            Option opt = cmd.getOption("oc");
-            if ("ISO-2022-JP".compareToIgnoreCase(opt.getValue()) == 0) {
-                options.put("output", NKFCharset.JIS);
-            } else if ("EUC-JP".compareToIgnoreCase(opt.getValue()) == 0) {
-                options.put("output", NKFCharset.EUC);
-            } else if ("CP932".compareToIgnoreCase(opt.getValue()) == 0) {
-                options.put("output", NKFCharset.SJIS);
-            } else if ("Shift_JIS".compareToIgnoreCase(opt.getValue()) == 0) {
-                options.put("output", NKFCharset.SJIS);
-            } else if ("Windows-31J".compareToIgnoreCase(opt.getValue()) == 0) {
-                options.put("output", NKFCharset.JIS);
-            } else if ("UTF-8".compareToIgnoreCase(opt.getValue()) == 0) {
-                options.put("output", NKFCharset.UTF8);
-            } else if ("UTF-8N".compareToIgnoreCase(opt.getValue()) == 0) {
-                options.put("output", NKFCharset.UTF8);
-            } else if ("UTF-16".compareToIgnoreCase(opt.getValue()) == 0) {
-                options.put("output", NKFCharset.UTF16);
-            } else if ("UTF-16BE-BOM".compareToIgnoreCase(opt.getValue()) == 0) {
-                options.put("output", NKFCharset.UTF16);
-            } else if ("UTF-32".compareToIgnoreCase(opt.getValue()) == 0) {
-                options.put("output", NKFCharset.UTF32);
-            } else if ("UTF-32BE-BOM".compareToIgnoreCase(opt.getValue()) == 0) {
-                options.put("output", NKFCharset.UTF32);
-            }
-        }
-        if (cmd.hasOption("ic")) {
-            Option opt = cmd.getOption("ic");
-            if ("ISO-2022-JP".compareToIgnoreCase(opt.getValue()) == 0) {
-                options.put("input", NKFCharset.JIS);
-            } else if ("EUC-JP".compareToIgnoreCase(opt.getValue()) == 0) {
-                options.put("input", NKFCharset.EUC);
-            } else if ("CP932".compareToIgnoreCase(opt.getValue()) == 0) {
-                options.put("input", NKFCharset.SJIS);
-            } else if ("Shift_JIS".compareToIgnoreCase(opt.getValue()) == 0) {
-                options.put("input", NKFCharset.SJIS);
-            } else if ("Windows-31J".compareToIgnoreCase(opt.getValue()) == 0) {
-                options.put("input", NKFCharset.SJIS);
-            } else if ("UTF-8".compareToIgnoreCase(opt.getValue()) == 0) {
-                options.put("input", NKFCharset.UTF8);
-            } else if ("UTF-8N".compareToIgnoreCase(opt.getValue()) == 0) {
-                options.put("input", NKFCharset.UTF8);
-            } else if ("UTF-16".compareToIgnoreCase(opt.getValue()) == 0) {
-                options.put("input", NKFCharset.UTF16);
-            } else if ("UTF-16BE-BOM".compareToIgnoreCase(opt.getValue()) == 0) {
-                options.put("input", NKFCharset.UTF16);
-            } else if ("UTF-32".compareToIgnoreCase(opt.getValue()) == 0) {
-                options.put("input", NKFCharset.UTF32);
-            } else if ("UTF-32BE-BOM".compareToIgnoreCase(opt.getValue()) == 0) {
-                options.put("input", NKFCharset.UTF32);
-            }
-        }
-
-        return options;
-    }
-
-    static abstract class Converter {
-
-        protected final ThreadContext context;
-        protected final Map<String, NKFCharset> options;
-
-        public Converter(ThreadContext ctx, Map<String, NKFCharset> opt) {
-            context = ctx;
-            options = opt;
-        }
-
-        static boolean isMimeText(ByteList str, Map<String, NKFCharset> options) {
-            if (str.length() <= 6) {
-                return false;
-            }
-            if (options.get("mime-decode") == NKFCharset.NOCONV) {
-                return false;
-            }
-            if (str.indexOf(BEGIN_MIME_STRING) < 0) {
-                return false;
-            }
-            if (str.lastIndexOf(END_MIME_STRING) < 0) {
-                return false;
-            }
-            return true;
-        }
-
-        private static RubyString encodeMimeString(Ruby runtime, RubyString str, ByteList format) {
-            RubyArray array = RubyArray.newArray(runtime, str);
-            return Pack.pack(runtime, array, format).chomp(runtime.getCurrentContext());
-        }
-
-        abstract RubyString convert(ByteList str);
-
-        ByteList convert_byte(ByteList str, String inputCharset, NKFCharset output) {
-            String outputCharset = output.getCharset();
-
-            if (inputCharset == null) {
-                inputCharset = str.getEncoding().toString();
-            }
-
-            if (outputCharset.equals(inputCharset)) {
-                return str.dup();
-            }
-
-            byte[] outCharsetBytes = outputCharset.getBytes();
-
-            EConv ec = EncodingUtils.econvOpenOpts(context, inputCharset.getBytes(), outCharsetBytes, 0, context.nil);
-
-            if (ec == null) {
-                throw context.runtime.newArgumentError("invalid encoding pair: " + inputCharset + " to " + outputCharset);
-            }
-
-            ByteList converted = EncodingUtils.econvStrConvert(context, ec, str, EConvFlags.INVALID_REPLACE);
-
-            converted.setEncoding(context.runtime.getEncodingService().findEncodingOrAliasEntry(outCharsetBytes).getEncoding());
-
-            return converted;
-        }
-    }
-
-    static class DefaultConverter extends Converter {
-
-        public DefaultConverter(ThreadContext ctx, Map<String, NKFCharset> opt) {
-            super(ctx, opt);
-        }
-
-        RubyString convert(ByteList str) {
-            NKFCharset input = options.get("input");
-            NKFCharset output = options.get("output");
-            ByteList b = convert_byte(str,
-                    input.getCharset(),
-                    output);
-            return context.runtime.newString(b);
-        }
-    }
-
-    static class MimeConverter extends Converter {
-
-        public MimeConverter(ThreadContext ctx, Map<String, NKFCharset> opt) {
-            super(ctx, opt);
-        }
-
-        private String detectCharset(String charset) {
-            if (charset.compareToIgnoreCase(NKFCharset.UTF8.getCharset()) == 0) {
-                return NKFCharset.UTF8.getCharset();
-            } else if (charset.compareToIgnoreCase(NKFCharset.JIS.getCharset()) == 0) {
-                return NKFCharset.JIS.getCharset();
-            } else if (charset.compareToIgnoreCase(NKFCharset.EUC.getCharset()) == 0) {
-                return NKFCharset.EUC.getCharset();
-            } else {
-                return NKFCharset.ASCII.getCharset();
-            }
-        }
-
-        private ByteList decodeMimeString(String str) {
-            String[] mime = str.split("^=\\?|\\?|\\?=$");
-            String charset = detectCharset(mime[1]);
-            int encode = mime[2].charAt(0);
-            ByteList body = new ByteList(mime[3].getBytes(), ASCIIEncoding.INSTANCE);
-
-            final RubyArray array;
-            if ('B' == encode || 'b' == encode) { // BASE64
-                array = Pack.unpack(context, body, PACK_BASE64);
-            } else { // Qencode
-                array = Pack.unpack(context, body, PACK_QENCODE);
-            }
-            RubyString s = (RubyString) array.entry(0);
-            ByteList decodeStr = s.asString().getByteList();
-
-            return convert_byte(decodeStr, charset, options.get("output"));
-        }
-
-        RubyString makeRubyString(ArrayList<ByteList> list) {
-            ByteList r = new ByteList();
-            for (ByteList l : list) {
-                r.append(l);
-            }
-            return context.runtime.newString(r);
-        }
-
-        RubyString convert(ByteList str) {
-            String s = Helpers.decodeByteList(context.runtime, str);
-            String[] token = s.split("\\s");
-            ArrayList<ByteList> raw_data = new ArrayList<ByteList>();
-
-            for (int i = 0; i < token.length; i++) {
-                raw_data.add(decodeMimeString(token[i]));
-            }
-
-            return makeRubyString(raw_data);
-        }
-
-    }
-
-    @Deprecated
-    public static final NKFCharset AUTO = NKFCharset.AUTO;
-    // no ISO-2022-JP in jcodings
-    @Deprecated
-    public static final NKFCharset JIS = NKFCharset.JIS;
-    @Deprecated
-    public static final NKFCharset EUC = NKFCharset.EUC;
-    @Deprecated
-    public static final NKFCharset SJIS = NKFCharset.SJIS;
-    @Deprecated
-    public static final NKFCharset BINARY = NKFCharset.BINARY;
-    @Deprecated
-    public static final NKFCharset NOCONV = NKFCharset.NOCONV;
-    @Deprecated
-    public static final NKFCharset UNKNOWN = NKFCharset.UNKNOWN;
-    @Deprecated
-    public static final NKFCharset ASCII = NKFCharset.ASCII;
-    @Deprecated
-    public static final NKFCharset UTF8 = NKFCharset.UTF8;
-    @Deprecated
-    public static final NKFCharset UTF16 = NKFCharset.UTF16;
-    @Deprecated
-    public static final NKFCharset UTF32 = NKFCharset.UTF32;
-    @Deprecated
-    public static final NKFCharset OTHER = NKFCharset.OTHER;
-    @Deprecated
-    public static final NKFCharset BASE64 = NKFCharset.BASE64;
-    @Deprecated
-    public static final NKFCharset QENCODE = NKFCharset.QENCODE;
-    @Deprecated
-    public static final NKFCharset MIME_DETECT = NKFCharset.MIME_DETECT;
-}
diff --git a/core/src/main/java/org/jruby/runtime/encoding/EncodingService.java b/core/src/main/java/org/jruby/runtime/encoding/EncodingService.java
index 0cbb43097e6b..863b353c0811 100644
--- a/core/src/main/java/org/jruby/runtime/encoding/EncodingService.java
+++ b/core/src/main/java/org/jruby/runtime/encoding/EncodingService.java
@@ -25,9 +25,7 @@
 import java.util.regex.Pattern;
 
 import org.jcodings.specific.USASCIIEncoding;
-import org.jruby.RubyFixnum;
 import org.jruby.RubyString;
-import org.jruby.ext.nkf.RubyNKF;
 import org.jruby.util.SafePropertyAccessor;
 import org.jruby.util.io.EncodingUtils;
 
@@ -284,11 +282,6 @@ private Encoding getEncodingFromObjectCommon(IRubyObject arg, boolean error) {
         if (arg instanceof RubyEncoding) {
             return ((RubyEncoding) arg).getEncoding();
         }
-        if (arg instanceof RubyFixnum) {
-            final int id = (int) arg.convertToInteger().getLongValue();
-            final String name = RubyNKF.NKFCharsetMap.get(id);
-            if ( name != null ) return getEncodingFromNKFName(name);
-        }
         if ( ( arg = arg.checkStringType() ).isNil() ) {
             return null;
         }
diff --git a/lib/pom.rb b/lib/pom.rb
index 4ebef188d868..90f5b06aa8f8 100644
--- a/lib/pom.rb
+++ b/lib/pom.rb
@@ -62,6 +62,7 @@ def log(message=nil)
     ['net-http', '0.4.0'],
     ['net-protocol', '0.2.2'],
     # Partial implementation in JRuby, unsure whether this is important
+    # Our ext is moving to the gem: https://github.com/ruby/nkf/issues/13
     # ['nkf', '0.1.1'],
     ['observer', '0.1.2'],
     ['open3', '0.2.1'],
diff --git a/lib/ruby/stdlib/kconv.rb b/lib/ruby/stdlib/kconv.rb
deleted file mode 100644
index f52b755288b7..000000000000
--- a/lib/ruby/stdlib/kconv.rb
+++ /dev/null
@@ -1,283 +0,0 @@
-# frozen_string_literal: false
-#
-# kconv.rb - Kanji Converter.
-#
-# $Id$
-#
-# ----
-#
-# kconv.rb implements the Kconv class for Kanji Converter.  Additionally,
-# some methods in String classes are added to allow easy conversion.
-#
-
-require 'nkf'
-
-#
-# Kanji Converter for Ruby.
-#
-module Kconv
-  #
-  # Public Constants
-  #
-
-  #Constant of Encoding
-
-  # Auto-Detect
-  AUTO = NKF::AUTO
-  # ISO-2022-JP
-  JIS = NKF::JIS
-  # EUC-JP
-  EUC = NKF::EUC
-  # Shift_JIS
-  SJIS = NKF::SJIS
-  # BINARY
-  BINARY = NKF::BINARY
-  # NOCONV
-  NOCONV = NKF::NOCONV
-  # ASCII
-  ASCII = NKF::ASCII
-  # UTF-8
-  UTF8 = NKF::UTF8
-  # UTF-16
-  UTF16 = NKF::UTF16
-  # UTF-32
-  UTF32 = NKF::UTF32
-  # UNKNOWN
-  UNKNOWN = NKF::UNKNOWN
-
-  #
-  # Public Methods
-  #
-
-  # call-seq:
-  #    Kconv.kconv(str, to_enc, from_enc=nil)
-  #
-  # Convert <code>str</code> to <code>to_enc</code>.
-  # <code>to_enc</code> and <code>from_enc</code> are given as constants of Kconv or Encoding objects.
-  def kconv(str, to_enc, from_enc=nil)
-    opt = ''
-    opt += ' --ic=' + from_enc.to_s if from_enc
-    opt += ' --oc=' + to_enc.to_s if to_enc
-
-    ::NKF::nkf(opt, str)
-  end
-  module_function :kconv
-
-  #
-  # Encode to
-  #
-
-  # call-seq:
-  #    Kconv.tojis(str)   => string
-  #
-  # Convert <code>str</code> to ISO-2022-JP
-  def tojis(str)
-    kconv(str, JIS)
-  end
-  module_function :tojis
-
-  # call-seq:
-  #    Kconv.toeuc(str)   => string
-  #
-  # Convert <code>str</code> to EUC-JP
-  def toeuc(str)
-    kconv(str, EUC)
-  end
-  module_function :toeuc
-
-  # call-seq:
-  #    Kconv.tosjis(str)   => string
-  #
-  # Convert <code>str</code> to Shift_JIS
-  def tosjis(str)
-    kconv(str, SJIS)
-  end
-  module_function :tosjis
-
-  # call-seq:
-  #    Kconv.toutf8(str)   => string
-  #
-  # Convert <code>str</code> to UTF-8
-  def toutf8(str)
-    kconv(str, UTF8)
-  end
-  module_function :toutf8
-
-  # call-seq:
-  #    Kconv.toutf16(str)   => string
-  #
-  # Convert <code>str</code> to UTF-16
-  def toutf16(str)
-    kconv(str, UTF16)
-  end
-  module_function :toutf16
-
-  # call-seq:
-  #    Kconv.toutf32(str)   => string
-  #
-  # Convert <code>str</code> to UTF-32
-  def toutf32(str)
-    kconv(str, UTF32)
-  end
-  module_function :toutf32
-
-  # call-seq:
-  #    Kconv.tolocale   => string
-  #
-  # Convert <code>self</code> to locale encoding
-  def tolocale(str)
-    kconv(str, Encoding.locale_charmap)
-  end
-  module_function :tolocale
-
-  #
-  # guess
-  #
-
-  # call-seq:
-  #    Kconv.guess(str)   => encoding
-  #
-  # Guess input encoding by NKF.guess
-  def guess(str)
-    ::NKF::guess(str)
-  end
-  module_function :guess
-
-  #
-  # isEncoding
-  #
-
-  # call-seq:
-  #    Kconv.iseuc(str)   => true or false
-  #
-  # Returns whether input encoding is EUC-JP or not.
-  #
-  # *Note* don't expect this return value is MatchData.
-  def iseuc(str)
-    str.dup.force_encoding(EUC).valid_encoding?
-  end
-  module_function :iseuc
-
-  # call-seq:
-  #    Kconv.issjis(str)   => true or false
-  #
-  # Returns whether input encoding is Shift_JIS or not.
-  def issjis(str)
-    str.dup.force_encoding(SJIS).valid_encoding?
-  end
-  module_function :issjis
-
-  # call-seq:
-  #    Kconv.isjis(str)   => true or false
-  #
-  # Returns whether input encoding is ISO-2022-JP or not.
-  def isjis(str)
-    /\A [\t\n\r\x20-\x7E]*
-      (?:
-        (?:\x1b \x28 I      [\x21-\x7E]*
-          |\x1b \x28 J      [\x21-\x7E]*
-          |\x1b \x24 @      (?:[\x21-\x7E]{2})*
-          |\x1b \x24 B      (?:[\x21-\x7E]{2})*
-          |\x1b \x24 \x28 D (?:[\x21-\x7E]{2})*
-        )*
-        \x1b \x28 B [\t\n\r\x20-\x7E]*
-      )*
-     \z/nox =~ str.dup.force_encoding('BINARY') ? true : false
-  end
-  module_function :isjis
-
-  # call-seq:
-  #    Kconv.isutf8(str)   => true or false
-  #
-  # Returns whether input encoding is UTF-8 or not.
-  def isutf8(str)
-    str.dup.force_encoding(UTF8).valid_encoding?
-  end
-  module_function :isutf8
-end
-
-class String
-  # call-seq:
-  #    String#kconv(to_enc, from_enc)
-  #
-  # Convert <code>self</code> to <code>to_enc</code>.
-  # <code>to_enc</code> and <code>from_enc</code> are given as constants of Kconv or Encoding objects.
-  def kconv(to_enc, from_enc=nil)
-    from_enc = self.encoding if !from_enc && self.encoding != Encoding.list[0]
-    Kconv::kconv(self, to_enc, from_enc)
-  end
-
-  #
-  # to Encoding
-  #
-
-  # call-seq:
-  #    String#tojis   => string
-  #
-  # Convert <code>self</code> to ISO-2022-JP
-  def tojis; Kconv.tojis(self) end
-
-  # call-seq:
-  #    String#toeuc   => string
-  #
-  # Convert <code>self</code> to EUC-JP
-  def toeuc; Kconv.toeuc(self) end
-
-  # call-seq:
-  #    String#tosjis   => string
-  #
-  # Convert <code>self</code> to Shift_JIS
-  def tosjis; Kconv.tosjis(self) end
-
-  # call-seq:
-  #    String#toutf8   => string
-  #
-  # Convert <code>self</code> to UTF-8
-  def toutf8; Kconv.toutf8(self) end
-
-  # call-seq:
-  #    String#toutf16   => string
-  #
-  # Convert <code>self</code> to UTF-16
-  def toutf16; Kconv.toutf16(self) end
-
-  # call-seq:
-  #    String#toutf32   => string
-  #
-  # Convert <code>self</code> to UTF-32
-  def toutf32; Kconv.toutf32(self) end
-
-  # call-seq:
-  #    String#tolocale   => string
-  #
-  # Convert <code>self</code> to locale encoding
-  def tolocale; Kconv.tolocale(self) end
-
-  #
-  # is Encoding
-  #
-
-  # call-seq:
-  #    String#iseuc   => true or false
-  #
-  # Returns whether <code>self</code>'s encoding is EUC-JP or not.
-  def iseuc;	Kconv.iseuc(self) end
-
-  # call-seq:
-  #    String#issjis   => true or false
-  #
-  # Returns whether <code>self</code>'s encoding is Shift_JIS or not.
-  def issjis;	Kconv.issjis(self) end
-
-  # call-seq:
-  #    String#isjis   => true or false
-  #
-  # Returns whether <code>self</code>'s encoding is ISO-2022-JP or not.
-  def isjis;	Kconv.isjis(self) end
-
-  # call-seq:
-  #    String#isutf8   => true or false
-  #
-  # Returns whether <code>self</code>'s encoding is UTF-8 or not.
-  def isutf8;	Kconv.isutf8(self) end
-end
diff --git a/lib/ruby/stdlib/nkf.rb b/lib/ruby/stdlib/nkf.rb
deleted file mode 100644
index 0103d672d42b..000000000000
--- a/lib/ruby/stdlib/nkf.rb
+++ /dev/null
@@ -1,2 +0,0 @@
-# Load built-in nkf library
-JRuby::Util.load_ext("org.jruby.ext.nkf.RubyNKF")
\ No newline at end of file