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1. Introduction

ISO/IEC 10646 [ISO.10646] defines a large character set called the Universal Character Set (UCS), which encompasses most of the world's writing systems. The same set of characters is defined by the Unicode standard [UNICODE], which further defines additional character properties and other application details of great interest to implementers. Up to the present time, changes in Unicode and amendments and additions to ISO/IEC 10646 have tracked each other, so that the character repertoires and code point assignments have remained in sync. The relevant standardization committees have committed to maintain this very useful synchronism.

Encoding Forms​

ISO/IEC 10646 and Unicode define several encoding forms of their common repertoire:

  • UTF-8
  • UCS-2
  • UTF-16
  • UCS-4
  • UTF-32

In an encoding form, each character is represented as one or more encoding units. All standard UCS encoding forms except UTF-8 have an encoding unit larger than one octet, making them hard to use in many current applications and protocols that assume 8 or even 7 bit characters.

Characteristics of UTF-8​

UTF-8, the object of this memo, has a one-octet encoding unit. It uses all bits of an octet, but has the quality of preserving the full US-ASCII [US-ASCII] range:

  • US-ASCII characters are encoded in one octet having the normal US-ASCII value
  • Any octet with such a value can only stand for a US-ASCII character, and nothing else

UTF-8 Encoding Properties​

UTF-8 encodes UCS characters as a varying number of octets, where the number of octets, and the value of each, depend on the integer value assigned to the character in ISO/IEC 10646 (the character number, a.k.a. code position, code point or Unicode scalar value).

This encoding form has the following characteristics (all values are in hexadecimal):

✅ ASCII Compatibility​

Character numbers from U+0000 to U+007F (US-ASCII repertoire) correspond to octets 00 to 7F (7 bit US-ASCII values). A direct consequence is that a plain ASCII string is also a valid UTF-8 string.

✅ No ASCII Confusion​

US-ASCII octet values do not appear otherwise in a UTF-8 encoded character stream. This provides compatibility with file systems or other software (e.g., the printf() function in C libraries) that parse based on US-ASCII values but are transparent to other values.

✅ Easy Conversion​

Round-trip conversion is easy between UTF-8 and other encoding forms.

✅ Sequence Length Indication​

The first octet of a multi-octet sequence indicates the number of octets in the sequence.

✅ Restricted Byte Values​

The octet values C0, C1, F5 to FF never appear.

✅ Character Boundary Recognition​

Character boundaries are easily found from anywhere in an octet stream.

✅ Sorting Preservation​

The byte-value lexicographic sorting order of UTF-8 strings is the same as if ordered by character numbers. Of course this is of limited interest since a sort order based on character numbers is almost never culturally valid.

✅ Fast Search Algorithms​

The Boyer-Moore fast search algorithm can be used with UTF-8 data.

✅ Recognizability​

UTF-8 strings can be fairly reliably recognized as such by a simple algorithm, i.e., the probability that a string of characters in any other encoding appears as valid UTF-8 is low, diminishing with increasing string length.

History of UTF-8​

UTF-8 was devised in September 1992 by Ken Thompson, guided by design criteria specified by Rob Pike, with the objective of defining a UCS transformation format usable in the Plan9 operating system in a non-disruptive manner. Thompson's design was stewarded through standardization by the X/Open Joint Internationalization Group XOJIG (see [FSS_UTF]), bearing the names FSS-UTF (variant FSS/UTF), UTF-2 and finally UTF-8 along the way.

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