Class: Encoding

Relationships & Source Files
Namespace Children
Inherits: Object
Defined in: encoding.c,


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:

#=> "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 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::UTF_8 after v2.0, 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 IO.new 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 IO.new 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:"
  p io.read

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 Attribute Summary

Class Method Summary

Instance Attribute Summary

Instance Method Summary

Class Attribute Details

.default_externalenc (rw)

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, if default_internal is not nil.

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

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# File 'encoding.c', line 1690

static VALUE
get_default_external(VALUE klass)
    return rb_enc_default_external();

.default_external=(enc) (rw)

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.

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# File 'encoding.c', line 1719

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

.default_internalenc (rw)

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 script encoding (__ENCODING__), not default_internal, is used as the encoding of created strings.

default_internal is initialized with -E option or nil otherwise.

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# File 'encoding.c', line 1773

static VALUE
get_default_internal(VALUE klass)
    return rb_enc_default_internal();

.default_internal=(enc or nil) (rw)

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.

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# File 'encoding.c', line 1799

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

Class Method Details


This method is for internal use only.
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# File 'encoding.c', line 1509

static VALUE
enc_load(VALUE klass, VALUE str)
    return str;


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

#=> {"BINARY"=>"ASCII-8BIT", "ASCII"=>"US-ASCII", "ANSI_X3.4-1968"=>"US-ASCII",
      "SJIS"=>"Windows-31J", "eucJP"=>"EUC-JP", "CP932"=>"Windows-31J"}
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# File 'encoding.c', line 1931

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];

.compatible?(obj1, obj2) ⇒ enc?

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

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# File 'encoding.c', line 1479

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);

.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.

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# File 'encoding.c', line 1444

static VALUE
enc_find(VALUE klass, VALUE enc)
    int idx;
    if (is_obj_encoding(enc))
	return enc;
    idx = str_to_encindex(enc);
    if (idx == UNSPECIFIED_ENCODING) return Qnil;
    return rb_enc_from_encoding_index(idx);

.listArray, ...

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)>]
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# File 'encoding.c', line 1407

static VALUE
enc_list(VALUE klass)
    VALUE ary = rb_ary_new2(0);

        rb_ary_replace(ary, rb_default_encoding_list);
        rb_ary_concat(ary, rb_additional_encoding_list);

    return ary;


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(Encoding.locale_charmap) may cause an error. If you need some encoding object even for unknown locale, .find(“locale”) can be used.

[ GitHub ]

# File 'localeinit.c', line 90

rb_locale_charmap(VALUE klass)
    return rb_usascii_str_new_cstr("US-ASCII");
    return locale_charmap(rb_usascii_str_new_cstr);

.name_listArray, ...

Returns the list of available encoding names.

#=> ["US-ASCII", "ASCII-8BIT", "UTF-8",
      "ISO-8859-1", "Shift_JIS", "EUC-JP",
      "BINARY", "CP932", "eucJP"]
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# File 'encoding.c', line 1881

static VALUE
rb_enc_name_list(VALUE klass)
    VALUE ary;

        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 Attribute Details

#ascii_compatible?Boolean (readonly)

Returns whether ASCII-compatible or not.

Encoding::UTF_8.ascii_compatible?     #=> true
Encoding::UTF_16BE.ascii_compatible?  #=> false
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# File 'encoding.c', line 676

static VALUE
enc_ascii_compatible_p(VALUE enc)
    return rb_enc_asciicompat(must_encoding(enc)) ? Qtrue : Qfalse;

#dummy?Boolean (readonly)

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
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# File 'encoding.c', line 660

static VALUE
enc_dummy_p(VALUE enc)
    return ENC_DUMMY_P(must_encoding(enc)) ? Qtrue : Qfalse;

Instance Method Details


This method is for internal use only.
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# File 'encoding.c', line 1501

static VALUE
enc_dump(int argc, VALUE *argv, VALUE self)
    rb_check_arity(argc, 0, 1);
    return enc_name(self);


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)>"
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# File 'encoding.c', line 1322

static VALUE
enc_inspect(VALUE self)
    rb_encoding *enc;

    if (!is_data_encoding(self)) {
    if (!(enc = DATA_PTR(self)) || rb_enc_from_index(rb_enc_to_index(enc)) != enc) {
	rb_raise(rb_eTypeError, "broken Encoding");
    return rb_enc_sprintf(rb_usascii_encoding(),
			  "#<%"PRIsVALUE":%s%s%s>", rb_obj_class(self),
			  (ENC_DUMMY_P(enc) ? " (dummy)" : ""),
			  enc_autoload_p(enc) ? " (autoload)" : "");

#nameString #to_sString

Alias for #to_s.


Returns the list of name and aliases of the encoding.

Encoding::WINDOWS_31J.names  #=> ["Windows-31J", "CP932", "csWindows31J", "SJIS", "PCK"]
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# File 'encoding.c', line 1375

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];

#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.

[ GitHub ]

# File 'encoding.c', line 566

static VALUE
enc_replicate_m(VALUE encoding, VALUE name)
    int idx = rb_enc_replicate(name_for_encoding(&name), rb_to_encoding(encoding));
    return rb_enc_from_encoding_index(idx);

#nameString #to_sString
Also known as: #name

Returns the name of the encoding.

Encoding::UTF_8.name      #=> "UTF-8"
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# File 'encoding.c', line 1349

static VALUE
enc_name(VALUE self)
    return rb_fstring_cstr(rb_enc_name((rb_encoding*)DATA_PTR(self)));