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To use Unicode in the domain part of email addresses, IDNA encoding must traditionally be used. Alternatively, SMTPUTF8 [3] allows the use of UTF-8 encoding in email addresses (both in a local part and in domain name) as well as in a mail header section. Various standards had been created to retrofit the handling of non-ASCII data to the ...
Some naming conventions limit whether letters may appear in uppercase or lowercase. Other conventions do not restrict letter case, but attach a well-defined interpretation based on letter case. Some naming conventions specify whether alphabetic, numeric, or alphanumeric characters may be used, and if so, in what sequence.
It ranges from U+0000 to U+007F, contains 128 characters and includes the C0 controls, ASCII punctuation and symbols, ASCII digits, both the uppercase and lowercase of the English alphabet and a control character.
The format of an email address is local-part@domain, where the local-part may be up to 64 octets long and the domain may have a maximum of 255 octets. [5] The formal definitions are in RFC 5322 (sections 3.2.3 and 3.4.1) and RFC 5321—with a more readable form given in the informational RFC 3696 (written by J. Klensin, the author of RFC 5321) and the associated errata.
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. A numeric character reference uses the format &#nnnn; or &#xhhhh; where nnnn is the code point in decimal form, and hhhh is the code point in hexadecimal form.
There is rarely anything to say about file extensions; instead, these should in almost all cases be redirects to an article about the file format, a list of file formats (e.g. for the many text-like formats) or a program using the file format (the sole or most common program, e.g. ".doc" → Microsoft Word).
This led to the idea that text in Chinese and other languages would take more space in UTF-8. However, text is only larger if there are more of these code points than 1-byte ASCII code points, and this rarely happens in the real-world documents due to spaces, newlines, digits, punctuation, English words, and (depending on document format) markup.
Files that contain machine-executable code and non-textual data typically contain all 256 possible eight-bit byte values. Many computer programs came to rely on this distinction between seven-bit text and eight-bit binary data, and would not function properly if non-ASCII characters appeared in data that was expected to include only ASCII text.