Plain Text
Last updated
Last updated
According to The Unicode Standard:
"Plain text is a pure sequence of character codes; plain Un-encoded text is therefore a sequence of Unicode character codes.
In contrast, styled text, also known as rich text, is any text representation containing plain text plus added information such as a language identifier, font size, color, hypertext links, and so on.
The near-ubiquity of ASCII was a great help, but failed to address international and linguistic concerns. The dollar-sign ("$") was not as useful in England, and the accented characters used in Spanish, French, German, Portuguese, and many other languages were entirely unavailable in ASCII (not to mention characters used in Greek, Russian, and most Eastern languages). Many individuals, companies, and countries defined extra characters as needed—often reassigning control characters, or using values in the range from 128 to 255. Using values above 128 conflicts with using the 8th bit as a checksum, but the checksum usage gradually died out.
Text is considered plain text regardless of its encoding. To properly understand or process it the recipient must know (or be able to figure out) what encoding was used; however, they need not know anything about the computer architecture that was used, or about the binary structures defined by whatever program (if any) created the data.
SGML, RTF, HTML, XML, and TEX are examples of rich text fully represented as plain text streams, interspersing plain text data with sequences of characters that represent the additional data structures."
According to other definitions, however, files that contain or other are generally considered plain text, so long as the markup is also in directly form (as in , , and so on). Thus, representations such as , , , , , and , as well as nearly all programming language source code files, are considered plain text. The particular content is irrelevant to whether a file is plain text. For example, an file can express drawings or even bitmapped graphics, but is still plain text.
The use of plain text rather than binary files enables files to survive much better "in the wild", in part by making them largely immune to computer architecture incompatibilities. For example, all the problems of can be avoided (with encodings such as rather than UTF-8, endianness matters, but uniformly for every character, rather than for potentially-unknown subsets of it).
The purpose of using plain text today is primarily independence from programs that require their very own special encoding or formatting or . Plain text files can be opened, read, and edited with ubiquitous and utilities.
A allows people to give commands in plain text and get a response, also typically in plain text.
Many other computer programs are also capable of processing or creating plain text, such as countless programs in , , , and and its kin; as well as web browsers (a few browsers such as and the produce only plain text for display) and other readers.
Plain text files are almost universal in programming; a source code file containing instructions in a is almost always a plain text file. Plain text is also commonly used for , which are read for saved settings at the startup of a program.
Plain text is used for much .
A , a "" file, or a generally contains only plain text (without formatting) intended for humans to read.
The best format for storing knowledge persistently is plain text, rather than some .
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Before the early 1960s, computers were mainly used for number-crunching rather than for text, and memory was extremely expensive. Computers often allocated only 6 bits for each character, permitting only 64 characters—assigning codes for A-Z, a-z, and 0-9 would leave only 2 codes: nowhere near enough. Most computers opted not to support lower-case letters. Thus, early text projects such as 's , the , and others had to resort to conventions such as keying an asterisk preceding letters actually intended to be upper-case.
of argued strongly for going to 8-bit bytes, because someday people might want to process text; and won. Although IBM used , most text from then on came to be encoded in , using values from 0 to 31 for (non-printing) , and values from 32 to 127 for graphic characters such as letters, digits, and punctuation. Most machines stored characters in 8 bits rather than 7, ignoring the remaining bit or using it as a .
These additional characters were encoded differently in different countries, making texts impossible to decode without figuring out the originator's rules. For instance, a browser might display ¬A rather than ` if it tried to interpret one character set as another. The International Organisation for Standardisation () eventually developed several under , to accommodate various languages. The first of these () is also known as "Latin-1", and covers the needs of most (not all) European languages that use Latin-based characters (there was not quite enough room to cover them all). then provided conventions for "switching" between different character sets in mid-file. Many other organisations developed variations on these, and for many years Windows and Macintosh computers used incompatible variations.
The text-encoding situation became more and more complex, leading to efforts by ISO and by the to develop a single, unified character encoding that could cover all known (or at least all currently known) languages. After some conflict,[] these efforts were unified. currently allows for 1,114,112 code values, and assigns codes covering nearly all modern text writing systems, as well as many historical ones, and for many non-linguistic characters such as printer's , mathematical symbols, etc.
Perhaps the most common way of explicitly stating the specific encoding of plain text is with a . For email and , the default MIME type is "" -- plain text without markup. Another MIME type often used in both email and HTTP is "; charset=UTF-8" -- plain text represented using the UTF-8 character encoding with HTML markup. Another common MIME type is "application/json" -- plain text represented using the UTF-8 character encoding with markup.
When a document is received without any explicit indication of the character encoding, some applications use to attempt to guess what encoding was used.
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reserves the first 32 codes (numbers 0–31 decimal) for known as the "C0 set": codes originally intended not to represent printable information, but rather to control devices (such as ) that make use of ASCII, or to provide about data streams such as those stored on magnetic tape. They include common characters like the and the .
In 8-bit character sets such as and the other sets, the first 32 characters of the "upper half" (128 to 159) are also control codes, known as the "C1 set". They are rarely used directly; when they turn up in documents which are ostensibly in an ISO 8859 encoding, their code positions generally refer instead to the characters at that position in a proprietary, system-specific encoding, such as or , that use the codes to instead provide additional graphic characters.
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defines additional control characters, including direction override characters (used to explicitly mark right-to-left writing inside left-to-right writing and the other way around) and to select alternate forms of , and other characters.
