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While ASCII text encoded using UTF-8 is backward compatible with ASCII, this is not true when Unicode Standard recommendations are ignored and a BOM is added. A BOM can confuse software that isn't prepared for it but can otherwise accept UTF-8, e.g. programming languages that permit non-ASCII bytes in string literals but not at the start of the ...
Punched tape with the word "Wikipedia" encoded in ASCII.Presence and absence of a hole represents 1 and 0, respectively; for example, W is encoded as 1010111.. Character encoding is the process of assigning numbers to graphical characters, especially the written characters of human language, allowing them to be stored, transmitted, and transformed using computers. [1]
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". [75] 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.
Binary data and text in any other encoding are likely to contain byte sequences that are invalid as UTF-8, so existence of such invalid sequences indicates the file is not UTF-8, while lack of invalid sequences is a very strong indication the text is UTF-8. Practically the only exception is text containing only ASCII-range bytes, as this may be ...
The sequence also has no meaning in any arrangement of UTF-32 encoding, so, in summary, it serves as a fairly reliable indication that the text stream is encoded as UTF-16 in big-endian byte order. Conversely, if the first two bytes are 0xFF, 0xFE, then the text stream may be assumed to be encoded as UTF-16LE because, read as a 16-bit little ...
Unicode also adopted UTF-16, but in Unicode terminology, the high-half zone elements become "high surrogates" and the low-half zone elements become "low surrogates". [clarification needed] Another encoding, UTF-32 (previously named UCS-4), uses four bytes (total 32 bits) to encode a single character of the codespace. UTF-32 thereby permits a ...
Although not strictly required, UTF-8 is usually also transfer encoded to avoid problems across seven-bit mail servers. MIME transfer encoding of UTF-8 makes it either unreadable as a plain text (in the case of base64) or, for some languages and types of text, heavily size inefficient (in the case of quoted-printable).
Text may be ambiguous as to what encoding it is in, for instance pure ASCII text is valid ASCII or ISO-8859-1 or CP1252 or UTF-8. "Tags" may indicate a document encoding, but when this is incorrect this may be silently corrected by display software (for instance the HTML spec says that the tag for ISO-8859-1 should be treated as CP1252), so ...