An Encoding instance represents a character encoding usable in Ruby. It is defined as a constant under the Encoding namespace. It has a name and optionally, aliases:
#=> #<Encoding:ISO-8859-1>

#=> ["ISO-8859-1", "ISO8859-1"]

Ruby methods dealing with encodings return or accept Encoding instances as arguments (when a method accepts an Encoding instance as an argument, it can be passed an Encoding name or alias instead).

"some string".encoding
#=> #<Encoding:UTF-8>

string = "some string".encode(Encoding::ISO_8859_1)
#=> "some string"
#=> #<Encoding:ISO-8859-1>

"some string".encode "ISO-8859-1"
#=> "some string"

Encoding::ASCII_8BIT is a special encoding that is usually used for a byte string, not a character string. But as the name insists, its characters in the range of ASCII are considered as ASCII characters. This is useful when you use ASCII-8BIT characters with other ASCII compatible characters.

Changing an encoding

The associated Encoding of a String can be changed in two different ways.

First, it is possible to set the Encoding of a string to a new Encoding without changing the internal byte representation of the string, with String#force_encoding. This is how you can tell Ruby the correct encoding of a string.

#=> "R\xC3\xA9sum\xC3\xA9"
#=> #<Encoding:ISO-8859-1>
#=> "R\u00E9sum\u00E9"

Second, it is possible to transcode a string, i.e. translate its internal byte representation to another encoding. Its associated encoding is also set to the other encoding. See String#encode for the various forms of transcoding, and the Encoding::Converter class for additional control over the transcoding process.

#=> "R\u00E9sum\u00E9"
#=> #<Encoding:UTF-8>
string = string.encode!(Encoding::ISO_8859_1)
#=> "R\xE9sum\xE9"
#=> #<Encoding::ISO-8859-1>

Script encoding

All Ruby script code has an associated Encoding which any String literal created in the source code will be associated to.

The default script encoding is Encoding::US-ASCII, but it can be changed by a magic comment on the first line of the source code file (or second line, if there is a shebang line on the first). The comment must contain the word coding or encoding, followed by a colon, space and the Encoding name or alias:

# encoding: UTF-8

"some string".encoding
#=> #<Encoding:UTF-8>

The __ENCODING__ keyword returns the script encoding of the file which the keyword is written:

# encoding: ISO-8859-1

#=> #<Encoding:ISO-8859-1>

ruby -K will change the default locale encoding, but this is not recommended. Ruby source files should declare its script encoding by a magic comment even when they only depend on US-ASCII strings or regular expressions.

Locale encoding

The default encoding of the environment. Usually derived from locale.

see ::locale_charmap, ::find('locale')

Filesystem encoding

The default encoding of strings from the filesystem of the environment. This is used for strings of file names or paths.

see ::find('filesystem')

External encoding

Each IO object has an external encoding which indicates the encoding that Ruby will use to read its data. By default Ruby sets the external encoding of an IO object to the default external encoding. The default external encoding is set by locale encoding or the interpreter -E option. ::default_external returns the current value of the external encoding.

#=> "UTF-8"
#=> #<Encoding:UTF-8>

$ ruby -E ISO-8859-1 -e "p Encoding.default_external"

$ LANG=C ruby -e 'p Encoding.default_external'

The default external encoding may also be set through ::default_external=, but you should not do this as strings created before and after the change will have inconsistent encodings. Instead use ruby -E to invoke ruby with the correct external encoding.

When you know that the actual encoding of the data of an IO object is not the default external encoding, you can reset its external encoding with IO#set_encoding or set it at IO object creation (see options).

Internal encoding

To process the data of an IO object which has an encoding different from its external encoding, you can set its internal encoding. Ruby will use this internal encoding to transcode the data when it is read from the IO object.

Conversely, when data is written to the IO object it is transcoded from the internal encoding to the external encoding of the IO object.

The internal encoding of an IO object can be set with IO#set_encoding or at IO object creation (see options).

The internal encoding is optional and when not set, the Ruby default internal encoding is used. If not explicitly set this default internal encoding is nil meaning that by default, no transcoding occurs.

The default internal encoding can be set with the interpreter option -E. ::default_internal returns the current internal encoding.

$ ruby -e 'p Encoding.default_internal'

$ ruby -E ISO-8859-1:UTF-8 -e "p [Encoding.default_external, \
[#<Encoding:ISO-8859-1>, #<Encoding:UTF-8>]

The default internal encoding may also be set through ::default_internal=, but you should not do this as strings created before and after the change will have inconsistent encodings. Instead use ruby -E to invoke ruby with the correct internal encoding.

IO encoding example

In the following example a UTF-8 encoded string “Ru00E9sumu00E9” is transcoded for output to ISO-8859-1 encoding, then read back in and transcoded to UTF-8:

string = "R\u00E9sum\u00E9"

open("transcoded.txt", "w:ISO-8859-1") do |io|

puts "raw text:"
p File.binread("transcoded.txt")

open("transcoded.txt", "r:ISO-8859-1:UTF-8") do |io|
  puts "transcoded text:"

While writing the file, the internal encoding is not specified as it is only necessary for reading. While reading the file both the internal and external encoding must be specified to obtain the correct result.

$ ruby t.rb
raw text:

transcoded text:
Class Public methods
Encoding.aliases → {"alias1" => "orig1", "alias2" => "orig2", ...}

Returns the hash of available encoding alias and original encoding name.

#=> {"BINARY"=>"ASCII-8BIT", "ASCII"=>"US-ASCII", "ANSI_X3.4-1986"=>"US-ASCII",
      "SJIS"=>"Shift_JIS", "eucJP"=>"EUC-JP", "CP932"=>"Windows-31J"}
static VALUE
rb_enc_aliases(VALUE klass)
    VALUE aliases[2];
    aliases[0] = rb_hash_new();
    aliases[1] = rb_ary_new();
    st_foreach(enc_table.names, rb_enc_aliases_enc_i, (st_data_t)aliases);
    return aliases[0];
Encoding.compatible?(obj1, obj2) → enc or nil

Checks the compatibility of two objects.

If the objects are both strings they are compatible when they are concatenatable. The encoding of the concatenated string will be returned if they are compatible, nil if they are not.

Encoding.compatible?("\xa1".force_encoding("iso-8859-1"), "b")
#=> #<Encoding:ISO-8859-1>

#=> nil

If the objects are non-strings their encodings are compatible when they have an encoding and:

  • Either encoding is US-ASCII compatible

  • One of the encodings is a 7-bit encoding

static VALUE
enc_compatible_p(VALUE klass, VALUE str1, VALUE str2)
    rb_encoding *enc;

    if (!enc_capable(str1)) return Qnil;
    if (!enc_capable(str2)) return Qnil;
    enc = rb_enc_compatible(str1, str2);
    if (!enc) return Qnil;
    return rb_enc_from_encoding(enc);
Encoding.default_external → enc

Returns default external encoding.

The default external encoding is used by default for strings created from the following locations:

While strings created from these locations will have this encoding, the encoding may not be valid. Be sure to check String#valid_encoding?.

File data written to disk will be transcoded to the default external encoding when written.

The default external encoding is initialized by the locale or -E option.

static VALUE
get_default_external(VALUE klass)
    return rb_enc_default_external();
Encoding.default_external = enc

Sets default external encoding. You should not set ::default_external in ruby code as strings created before changing the value may have a different encoding from strings created after the value was changed., instead you should use ruby -E to invoke ruby with the correct default_external.

See ::default_external for information on how the default external encoding is used.

static VALUE
set_default_external(VALUE klass, VALUE encoding)
    rb_warning("setting Encoding.default_external");
    return encoding;
Encoding.default_internal → enc

Returns default internal encoding. Strings will be transcoded to the default internal encoding in the following places if the default internal encoding is not nil:

Additionally String#encode and String#encode! use the default internal encoding if no encoding is given.

The locale encoding (__ENCODING__), not ::default_internal, is used as the encoding of created strings.

::default_internal is initialized by the source file's internal_encoding or -E option.

static VALUE
get_default_internal(VALUE klass)
    return rb_enc_default_internal();
Encoding.default_internal = enc or nil

Sets default internal encoding or removes default internal encoding when passed nil. You should not set ::default_internal in ruby code as strings created before changing the value may have a different encoding from strings created after the change. Instead you should use ruby -E to invoke ruby with the correct default_internal.

See ::default_internal for information on how the default internal encoding is used.

static VALUE
set_default_internal(VALUE klass, VALUE encoding)
    rb_warning("setting Encoding.default_internal");
    return encoding;
Encoding.find(string) → enc

Search the encoding with specified name. name should be a string.

Encoding.find("US-ASCII")  #=> #<Encoding:US-ASCII>

Names which this method accept are encoding names and aliases including following special aliases


default external encoding


default internal encoding


locale encoding


filesystem encoding

An ArgumentError is raised when no encoding with name. Only Encoding.find("internal") however returns nil when no encoding named “internal”, in other words, when Ruby has no default internal encoding.

static VALUE
enc_find(VALUE klass, VALUE enc)
    int idx;
    if (RB_TYPE_P(enc, T_DATA) && is_data_encoding(enc))
        return enc;
    idx = str_to_encindex(enc);
    if (idx == UNSPECIFIED_ENCODING) return Qnil;
    return rb_enc_from_encoding_index(idx);
Encoding.list → [enc1, enc2, ...]

Returns the list of loaded encodings.

#=> [#<Encoding:ASCII-8BIT>, #<Encoding:UTF-8>,
      #<Encoding:ISO-2022-JP (dummy)>]

#=> #<Encoding:US-ASCII>

#=> [#<Encoding:ASCII-8BIT>, #<Encoding:UTF-8>,
      #<Encoding:US-ASCII>, #<Encoding:ISO-2022-JP (dummy)>]
static VALUE
enc_list(VALUE klass)
    VALUE ary = rb_ary_new2(0);
    rb_ary_replace(ary, rb_encoding_list);
    return ary;
Encoding.locale_charmap → string

Returns the locale charmap name. It returns nil if no appropriate information.

Debian GNU/Linux
    Encoding.locale_charmap  #=> "ANSI_X3.4-1968"
    Encoding.locale_charmap  #=> "EUC-JP"

SunOS 5
    Encoding.locale_charmap  #=> "646"
    Encoding.locale_charmap  #=> "eucJP"

The result is highly platform dependent. So ::find may cause an error. If you need some encoding object even for unknown locale, ::find(“locale”) can be used.

rb_locale_charmap(VALUE klass)
    return rb_usascii_str_new2("ASCII-8BIT");
#elif defined _WIN32 || defined __CYGWIN__
    const char *codeset = 0;
    char cp[sizeof(int) * 3 + 4];
# ifdef __CYGWIN__
    const char *nl_langinfo_codeset(void);
    codeset = nl_langinfo_codeset();
# endif
    if (!codeset) {
        UINT codepage = GetConsoleCP();
        if (!codepage) codepage = GetACP();
        snprintf(cp, sizeof(cp), "CP%d", codepage);
        codeset = cp;
    return rb_usascii_str_new2(codeset);
#elif defined HAVE_LANGINFO_H
    char *codeset;
    codeset = nl_langinfo(CODESET);
    return rb_usascii_str_new2(codeset);
    return Qnil;
Encoding.name_list → ["enc1", "enc2", ...]

Returns the list of available encoding names.

#=> ["US-ASCII", "ASCII-8BIT", "UTF-8",
      "ISO-8859-1", "Shift_JIS", "EUC-JP",
      "BINARY", "CP932", "eucJP"]
static VALUE
rb_enc_name_list(VALUE klass)
    VALUE ary = rb_ary_new2(enc_table.names->num_entries);
    st_foreach(enc_table.names, rb_enc_name_list_i, (st_data_t)ary);
    return ary;
Instance Public methods
enc.ascii_compatible? → true or false

Returns whether ASCII-compatible or not.

Encoding::UTF_8.ascii_compatible?     #=> true
Encoding::UTF_16BE.ascii_compatible?  #=> false
static VALUE
enc_ascii_compatible_p(VALUE enc)
    return rb_enc_asciicompat(enc_table.list[must_encoding(enc)].enc) ? Qtrue : Qfalse;
enc.dummy? → true or false

Returns true for dummy encodings. A dummy encoding is an encoding for which character handling is not properly implemented. It is used for stateful encodings.

Encoding::ISO_2022_JP.dummy?       #=> true
Encoding::UTF_8.dummy?             #=> false
static VALUE
enc_dummy_p(VALUE enc)
    return ENC_DUMMY_P(enc_table.list[must_encoding(enc)].enc) ? Qtrue : Qfalse;
enc.inspect → string

Returns a string which represents the encoding for programmers.

Encoding::UTF_8.inspect       #=> "#<Encoding:UTF-8>"
Encoding::ISO_2022_JP.inspect #=> "#<Encoding:ISO-2022-JP (dummy)>"
static VALUE
enc_inspect(VALUE self)
    VALUE str = rb_sprintf("#<%s:%s%s>", rb_obj_classname(self),
                      (enc_dummy_p(self) ? " (dummy)" : ""));
    ENCODING_CODERANGE_SET(str, rb_usascii_encindex(), ENC_CODERANGE_7BIT);
    return str;
} → string

Returns the name of the encoding.      #=> "UTF-8"
static VALUE
enc_name(VALUE self)
    return rb_usascii_str_new2(rb_enc_name((rb_encoding*)DATA_PTR(self)));
enc.names → array

Returns the list of name and aliases of the encoding.

Encoding::WINDOWS_31J.names  #=> ["Windows-31J", "CP932", "csWindows31J"]
static VALUE
enc_names(VALUE self)
    VALUE args[2];

    args[0] = (VALUE)rb_to_encoding_index(self);
    args[1] = rb_ary_new2(0);
    st_foreach(enc_table.names, enc_names_i, (st_data_t)args);
    return args[1];
enc.replicate(name) → encoding

Returns a replicated encoding of enc whose name is name. The new encoding should have the same byte structure of enc. If name is used by another encoding, raise ArgumentError.

static VALUE
enc_replicate(VALUE encoding, VALUE name)
    return rb_enc_from_encoding_index(
} → string

Returns the name of the encoding.      #=> "UTF-8"
static VALUE
enc_name(VALUE self)
    return rb_usascii_str_new2(rb_enc_name((rb_encoding*)DATA_PTR(self)));