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It was designed for backward compatibility with ASCII: the first 128 characters of Unicode, which correspond one-to-one with ASCII, are encoded using a single byte with the same binary value as ASCII, so that a UTF-8-encoded file using only those characters is identical to an ASCII file.
A UTF-8 file that contains only ASCII characters is identical to an ASCII file. Legacy programs can generally handle UTF-8 encoded files, even if they contain non-ASCII characters. For instance, the C printf function can print a UTF-8 string because it only looks for the ASCII '%' character to define a formatting string. All other bytes are ...
HTML and XML provide ways to reference Unicode characters when the characters themselves either cannot or should not be used. A numeric character reference refers to a character by its Universal Character Set/Unicode code point, and a character entity reference refers to a character by a predefined name.
The Basic Latin Unicode block, [3] sometimes informally called C0 Controls and Basic Latin, [4] is the first block of the Unicode standard, and the only block which is encoded in one byte in UTF-8. The block contains all the letters and control codes of the ASCII encoding.
The same character converted to UTF-8 becomes the byte sequence EF BB BF. The Unicode Standard allows the BOM "can serve as a signature for UTF-8 encoded text where the character set is unmarked". [74] Some software developers have adopted it for other encodings, including UTF-8, in an attempt to distinguish UTF-8 from local 8-bit code pages.
While ASCII is limited to 128 characters, Unicode and the UCS support more characters by separating the concepts of unique identification (using natural numbers called code points) and encoding (to 8-, 16-, or 32-bit binary formats, called UTF-8, UTF-16, and UTF-32, respectively).
The Universal Coded Character Set (UCS, Unicode) is a standard set of characters defined by the international standard ISO/IEC 10646, Information technology — Universal Coded Character Set (UCS) (plus my amendments to that standard), which is the basis of many character encodings, improving as characters from previously unrepresented typing systems are added.
The table on the left contains the actual Unicode characters; the one on the right contains the equivalents using HTML markup for the subscript or superscript. Unicode characters 0