String.encoding / encode / force_encoding

# Returns the current encoding of the string.
string.encoding

# Returns a new string converted to the specified encoding.
string.encode(encoding)
string.encode(destination, source)
string.encode(encoding, invalid: :replace, undef: :replace)

# Changes only the encoding label without converting the byte sequence.
string.force_encoding(encoding)

# Check the encoding of a string.
text = "Hello"
puts text.encoding  # UTF-8

# Convert from UTF-8 to Shift_JIS.
sjis = text.encode("Shift_JIS")
puts sjis.encoding  # Shift_JIS

# Replace characters that cannot be converted with a substitute character.
special = "Hello \u{1F600}"  # String containing an emoji
begin
  sjis = special.encode("Shift_JIS")
rescue Encoding::UndefinedConversionError => e
  puts "Conversion error: #{e.message}"
end

# Use the invalid/undef options to replace unconvertible characters.
safe = special.encode(
  "Shift_JIS",
  invalid: :replace,
  undef: :replace,
  replace: "?"
)
puts safe.encode("UTF-8")  # Hello ?

# Use force_encoding to change only the encoding label.
bytes = "\x82\xb1\x82\xf1\x82\xc9\x82\xbf\x82\xcd"
text2 = bytes.force_encoding("Shift_JIS")
puts text2.encode("UTF-8")  # こんにちは

Since Ruby 1.9, strings carry encoding information. Mixing strings with different encodings raises an Encoding::CompatibilityError. Modern Ruby programs typically use UTF-8, so you rarely need to think about encoding in day-to-day code — but it is worth checking when reading external files or exchanging data with APIs.

encode and force_encoding behave differently. encode converts the byte sequence, whereas force_encoding only changes how Ruby interprets the string without touching the bytes. Using them incorrectly can cause garbled text.

For type conversion of strings, see to_i / to_f / to_sym. For string length in bytes, see length / size / bytesize.