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Quoted-printable
Quoted-Printable, or QP encoding, is an encoding using printable ASCII characters (alphanumeric and the equals sign "=") to transmit 8-bit data over a 7-bit data path or, generally, over a medium which is not 8-bit clean.〔Historically, e-mail was often referred as non-8-bit-clean, because various media were used to transfer messages, sometimes other than Internet. Modern ESMTP servers are 8-bit clean though in most cases, see 8BITMIME.〕 It is defined as a MIME content transfer encoding for use in e-mail.
QP works by using the equals sign "=" as an escape character. It also limits line length to 76, as some software has limits on line length.
==Introduction==
MIME defines mechanisms for sending other kinds of information in e-mail, including text in languages other than English, using character encodings other than ASCII. However, these encodings often use byte values outside the ASCII range so they need to be encoded further before they are suitable for use in a non-8-bit-clean environment. Quoted-Printable encoding is one method used for mapping arbitrary bytes into sequences of ASCII characters. So, Quoted-Printable is not a character encoding scheme itself, but a data coding layer to be used under some byte-oriented character encoding. QP encoding is reversible, meaning the original bytes and hence the non-ASCII characters they represent can be identically recovered.
Quoted-Printable and Base64 are the two basic MIME content transfer encodings, if a trivial "8bit" encoding is not counted. If the text to be encoded does not contain many non-ASCII characters, then Quoted-Printable results in a fairly readable〔This implies that an ASCII compatible encoding is used. A QP-encoded text in e.g. EBCDIC would not be readable of course.〕 and compact encoded result. On the other hand, if the input is not mostly ASCII, then Quoted-Printable becomes both unreadable and extremely inefficient. Base64 is not human-readable, but has a uniform overhead for all data and is the more sensible choice for binary formats or text in non-Latin based languages.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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