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 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::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 Encoding.locale_charmap, Encoding.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 Encoding.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. Encoding.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 Encoding.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. Encoding.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 Encoding.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 Methods

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"}

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

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 -E option. If -E isn’t set, it is initialized to UTF-8 on Windows and the locale on other operating systems.

Sets default external encoding. You should not set Encoding::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 Encoding::default_external for information on how the default external encoding is used.

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.

Encoding::default_internal is initialized with -E option or nil otherwise.

Sets default internal encoding or removes default internal encoding when passed nil. You should not set Encoding::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 Encoding::default_internal for information on how the default internal encoding is used.

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.

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)>]

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

Returns the list of available encoding names.

#=> ["US-ASCII", "ASCII-8BIT", "UTF-8",
      "ISO-8859-1", "Shift_JIS", "EUC-JP",
      "BINARY", "CP932", "eucJP"]
Instance Methods

Returns whether ASCII-compatible or not.

Encoding::UTF_8.ascii_compatible?     #=> true
Encoding::UTF_16BE.ascii_compatible?  #=> 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

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)>"

Returns the list of name and aliases of the encoding.

Encoding::WINDOWS_31J.names  #=> ["Windows-31J", "CP932", "csWindows31J", "SJIS", "PCK"]

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.

Returns the name of the encoding.      #=> "UTF-8"