Specials is a short Unicode block of characters allocated at the very end of the Basic Multilingual Plane, at U+FFF0–FFFF, containing these code points: , marks start of annotated text , marks start of annotating character(s) , marks end of annotation block , placeholder in the text for another unspecified object, for example in a compound document. used to replace an unknown, unrecognised, or unrepresentable character not a character. not a character.

and are noncharacters, meaning they are reserved but do not cause ill-formed Unicode text. Versions of the Unicode standard from 3.1.0 to 6.3.0 claimed that these characters should never be interchanged, leading some applications to use them to guess text encoding by interpreting the presence of either as a sign that the text is not Unicode. However, Corrigendum #9 later specified that noncharacters are not illegal and so this method of checking text encoding is incorrect. An example of an internal usage of U+FFFE is the CLDR algorithm; this extended Unicode algorithm maps the noncharacter to a minimal, unique primary weight.

Unicode's character can be inserted at the beginning of a Unicode text as a byte order mark to signal its endianness: a program reading a text encoded in for example UTF-16 and encountering would then know that it should switch the byte order for all the following characters.

Its block name in Unicode 1.0 was Special.

Replacement character

The replacement character � (often displayed as a rhombus with a question mark) is a symbol found in the Unicode standard at code point U+FFFD in the Specials table. It is used to indicate problems when a system is unable to render a stream of data to correct symbols.

At one time, the replacement character was often used when there was no glyph available in a font for that character, as in font substitution. However, most modern text rendering systems instead use a font's character, which in most cases is an empty box, or a ? or X in a box (the browser rendering this page displays 􏿾 for U+10FFEE), sometimes called a "tofu". There is no Unicode code point for this symbol.

Thus the replacement character is now only seen for encoding errors.

As an example, an E-mail message that is encoded in Windows-1252 and contains non-breaking spaces in it will contain the byte 0xA0 for each non-breaking space. The message is then opened in a client that assumes the input is UTF-8. Since bytes between 0x80 and 0xFF are not valid as standalone bytes in UTF-8, the client opening this message could replace these bytes with the replacement character to produce a valid string of Unicode code points for display.

The replacement character () may get written out if the viewer saves or copies the text; these three bytes will then replace every encoding error. Even if the result is viewed as Windows-1252, these will now all display identically as �. As all UTF-8 encoding errors are turned into the same sequence, it is impossible to recover the original text. Some software programs translate invalid UTF-8 bytes to matching characters in Windows-1252 (since that is the most common source of these errors) or into , so less information is lost, and the result that the replacement character is never seen.

Unicode chart

History The following Unicode-related documents record the purpose and process of defining specific characters in the Specials block:

Version Count UTC ID L2 ID WG2 ID Document 1.0.0 U+FFFD 1 (to be determined) U+FFFE..FFFF 2 (to be determined) doc) N2403 2.1 U+FFFC 1 N1365 N1353 N1603 N1681 N1894 3.0 U+FFF9..FFFB 3 N1727 N1703 html) N1861 doc) N1920 html, doc) doc)

See also Unicode control characters

References