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In
computer science Computer science is the study of computation, automation, and information. Computer science spans theoretical disciplines (such as algorithms, theory of computation, information theory, and automation) to practical disciplines (includi ...
, an escape sequence is a combination of
characters Character or Characters may refer to: Arts, entertainment, and media Literature * ''Character'' (novel), a 1936 Dutch novel by Ferdinand Bordewijk * ''Characters'' (Theophrastus), a classical Greek set of character sketches attributed to The ...
that has a meaning other than the literal characters contained therein; it is marked by one or more preceding (and possibly terminating) characters.


Examples

* In C and many derivative programming languages, a string escape sequence is a series of two or more characters, starting with a backslash \. ** Note that in C a backslash immediately followed by a newline does not constitute an escape sequence, but splices physical source lines into logical ones in the second translation phase, whereas string escape sequences are converted in the fifth translation phase. ** To represent the backslash character itself, \\ can be used, whereby the first backslash indicates an escape and the second specifies that a backslash is being escaped. ** A character may be escaped in multiple different ways. Assuming ASCII encoding, the escape sequences \x5c ( hexadecimal), \\, \134 (
octal The octal numeral system, or oct for short, is the base-8 number system, and uses the digits 0 to 7. This is to say that 10octal represents eight and 100octal represents sixty-four. However, English, like most languages, uses a base-10 number ...
) and \x5C all encode the same character: the backslash \. * For devices that respond to ANSI escape sequences, the combination of three or more characters beginning with the ASCII "escape" character (decimal character code 27) followed by the left-bracket character
characters_ Character_or_Characters_may_refer_to: __Arts,_entertainment,_and_media__Literature *__''Character''_(novel),_a_1936_Dutch_novel_by_Ferdinand_Bordewijk *__''Characters''_(Theophrastus),_a_classical_Greek_set_of_character_sketches_attributed_to_The_...
_is_used_to_change_the_State_(computer_science).html" "title="character_(computing).html" "title="/code> (decimal character code 91) defines an escape sequence.


Control sequences

When directed this series of character (computing)">characters Character or Characters may refer to: Arts, entertainment, and media Literature * ''Character'' (novel), a 1936 Dutch novel by Ferdinand Bordewijk * ''Characters'' (Theophrastus), a classical Greek set of character sketches attributed to The ...
is used to change the State (computer science)">state State may refer to: Arts, entertainment, and media Literature * ''State Magazine'', a monthly magazine published by the U.S. Department of State * ''The State'' (newspaper), a daily newspaper in Columbia, South Carolina, United States * ''Our S ...
 of computers and their attached peripheral devices, rather than to be displayed or printed as regular Data (computing), data bytes would be, these are also known as control sequences, reflecting their use in device control, beginning with the Control Sequence Initiator - originally the "escape character" ASCII code - character 27 (decimal) - often written "Esc" on
keycap A keycap is a small cover of plastic, metal, or other material placed over the keyswitch of a computer keyboard. Keycaps are often illustrated to indicate the key function or alphanumeric Alphanumericals or alphanumeric characters are a co ...
s. With the introduction of ANSI terminals most escape sequences began with the ''two'' characters "ESC" then " Hayes-compatible modems * Data General terminal control sequences, but they often were still called escape sequences, and the very common use of "escaping" special characters in programming languages and command-line parameters today often use the "backslash" character to begin the sequence. Escape sequences in communications are commonly used when a computer and a peripheral have only a single channel through which to send information back and forth (so escape sequences are an example of
in-band signaling In telecommunications, in-band signaling is the sending of control information within the same band or channel used for data such as voice or video. This is in contrast to out-of-band signaling which is sent over a different channel, or even o ...
). They were common when most
dumb terminals A computer terminal is an electronic or electromechanical hardware device that can be used for entering data into, and transcribing data from, a computer or a computing system. The teletype was an example of an early-day hard-copy terminal and ...
used
ASCII ASCII ( ), abbreviated from American Standard Code for Information Interchange, is a character encoding standard for electronic communication. ASCII codes represent text in computers, telecommunications equipment, and other devices. Because ...
with 7 data bits for communication, and sometimes would be used to switch to a different character set for "foreign" or graphics characters that would otherwise been restricted by the 128 codes available in 7 data bits. Even relatively "dumb" terminals responded to some escape sequences, including the original mechanical Teletype printers (on which "glass Teletypes" or VDUs were based) responded to characters 27 and 31 to alternate between letters and figures modes.


Keyboard

An escape character is usually assigned to the
Esc key On computer keyboards, the Esc key (named ''Escape key'' in the international standard series ISO/IEC 9995) is a key used to generate the escape character (which can be represented as ASCII code 27 in decimal, Unicode U+001B, or ). The escape ...
on a
computer keyboard A computer keyboard is a peripheral input device modeled after the typewriter keyboard which uses an arrangement of buttons or keys to act as mechanical levers or electronic switches. Replacing early punched cards and paper tape technolog ...
, and can be sent in other ways than as part of an escape sequence. For example, the Esc key may be used as an input character in editors such as vi, or for backing up one level in a menu in some applications. The Hewlett Packard
HP 2640 The HP 2640A and other HP 264X models were block-mode "smart" and intelligent ASCII standard serial terminals produced by Hewlett-Packard using the Intel 8008 and 8080 microprocessors. History The HP 2640A was introduced in November 1974 at a ...
terminals had a key for a "display functions" mode which would display graphics for all control characters, including Esc, to aid in debugging applications. If the Esc key and other keys that send escape sequences are both supposed to be meaningful to an application, an ambiguity arises if a character terminal is in use. When the application receives the
ASCII ASCII ( ), abbreviated from American Standard Code for Information Interchange, is a character encoding standard for electronic communication. ASCII codes represent text in computers, telecommunications equipment, and other devices. Because ...
escape character, it is not clear whether that character is the result of the user pressing the Esc key or whether it is the initial character of an escape sequence (e.g., resulting from an arrow key press). The traditional method of resolving the ambiguity is to observe whether or not another character quickly follows the escape character. If not, it is assumed not to be part of an escape sequence. This
heuristic A heuristic (; ), or heuristic technique, is any approach to problem solving or self-discovery that employs a practical method that is not guaranteed to be optimal, perfect, or rational, but is nevertheless sufficient for reaching an immediate ...
can fail under some circumstances, especially without fast modern communication speeds. Escape sequences date back at least to the 1874
Baudot code The Baudot code is an early character encoding for telegraphy invented by Émile Baudot in the 1870s. It was the predecessor to the International Telegraph Alphabet No. 2 (ITA2), the most common teleprinter code in use until the advent of ASCII ...
.


Modem control

The
Hayes command set The Hayes command set (also known as the AT command set) is a specific command language originally developed by Dennis Hayes for the Hayes Smartmodem 300 baud modem in 1981. The command set consists of a series of short text strings which can be ...
, for instance, defines a single escape sequence, '' +++''. (In order to interpret ''+++'', which may be a part of data, as the escape sequence, the sender stops communication for one second before and after the ''+++''.) When the modem encounters this in a stream of data, it switches from its normal mode of operation, which simply sends any characters to the phone, to a command mode in which the following data is assumed to be a part of the command language. You can switch back to the ''online mode'' by sending the O command. The Hayes command set is modal, switching from command mode to online mode. This is not appropriate in the case where the commands and data will switch back and forth rapidly. An example of a non-modal escape sequence control language is the
VT100 The VT100 is a video terminal, introduced in August 1978 by Digital Equipment Corporation (DEC). It was one of the first terminals to support ANSI escape codes for cursor control and other tasks, and added a number of extended codes for special ...
, which used a series of commands prefixed by a
Control Sequence Introducer ANSI escape sequences are a standard for in-band signaling to control cursor location, color, font styling, and other options on video text terminals and terminal emulators. Certain sequences of bytes, most starting with an ASCII escape charac ...
.


Comparison with control characters

A control character is a character that, in isolation, has some control function, such as
carriage return A carriage return, sometimes known as a cartridge return and often shortened to CR, or return, is a control character or mechanism used to reset a device's position to the beginning of a line of text. It is closely associated with the line feed ...
(CR). Escape sequences, by contrast, consist of one or more escape characters which change the interpretation of subsequent characters.


ASCII video data terminals

The
VT52 The VT50 was a CRT-based computer terminal introduced by Digital Equipment Corporation (DEC) in July 1974. It provided a display with 12 rows and 80 columns of upper-case text, and used an expanded set of control characters and forward-only scro ...
terminal used simple digraph commands like escape-A: in isolation, "A" simply meant the letter "A", but as part of the escape sequence "escape-A", it had a different meaning. The VT52 also supported parameters: it was not a straightforward control language encoded as substitution. The later
VT100 The VT100 is a video terminal, introduced in August 1978 by Digital Equipment Corporation (DEC). It was one of the first terminals to support ANSI escape codes for cursor control and other tasks, and added a number of extended codes for special ...
terminal implemented the more sophisticated
ANSI escape sequences ANSI escape sequences are a standard for in-band signaling to control cursor location, color, font styling, and other options on video text terminals and terminal emulators. Certain sequences of bytes, most starting with an ASCII escape charac ...
standard (now ECMA-48) for functions such as controlling cursor movement, character set, and display enhancements. The Hewlett Packard
HP 2640 The HP 2640A and other HP 264X models were block-mode "smart" and intelligent ASCII standard serial terminals produced by Hewlett-Packard using the Intel 8008 and 8080 microprocessors. History The HP 2640A was introduced in November 1974 at a ...
series had perhaps the most elaborate escape sequences for block and character modes, programming keys and their soft labels, graphics vectors, and even saving data to tape or disk files.


Use in DOS and Windows

A utility,
ANSI.SYS ANSI.SYS is a device driver in the DOS family of operating systems that provides extra console functions through ANSI escape sequences. It is partially based upon a subset of the text terminal control standard proposed by the ANSI X3L2 Technical C ...
, can be used to enable the interpreting of the ANSI (ECMA-48) terminal escape sequences under
DOS DOS is shorthand for the MS-DOS and IBM PC DOS family of operating systems. DOS may also refer to: Computing * Data over signalling (DoS), multiplexing data onto a signalling channel * Denial-of-service attack (DoS), an attack on a communicat ...
(by using $e in the PROMPT command) or in command windows in 16-bit
Windows Windows is a group of several proprietary graphical operating system families developed and marketed by Microsoft. Each family caters to a certain sector of the computing industry. For example, Windows NT for consumers, Windows Server for ser ...
. The rise of
GUI The GUI ( "UI" by itself is still usually pronounced . or ), graphical user interface, is a form of user interface that allows users to interact with electronic devices through graphical icons and audio indicator such as primary notation, inste ...
applications, which directly write to display cards, has greatly reduced the usage of escape sequences on Microsoft platforms, but they can still be used to create interactive random-access character-based screen interfaces with the character-based library routines such as
printf The printf format string is a control parameter used by a class of functions in the input/output libraries of C and many other programming languages. The string is written in a simple template language: characters are usually copied literal ...
without resorting to a GUI program.


Use in Linux and Unix displays

The default text terminal, and text windows (such as using
xterm In computing, xterm is the standard terminal emulator for the X Window System. It allows users to run programs which require a command-line interface. If no particular program is specified, xterm runs the user's shell. An X display can sho ...
) respond to ANSI escape sequences.


Quoting escape


Overview

When an escape character is needed within the quoted/escaped string, there are two strategies used within programming and scripting languages: * doubled delimiter (e.g. 'He didn''t do it.') * secondary escape sequence An example of the latter is in the use of the caret (^). E.g. this outputs "You can do so via Cut&Paste" in CMD. (otherwise, the ampersand has a restricted use) echo You can do so via Cut^&Paste


In detail

A common use of escape sequences is in fact to remove control characters found in a binary data stream so that they will not cause their control function by mistake. In this case, the control character is replaced by a defined "escape character" (which need not be the US-ASCII escape character) and one or more other characters; after exiting the context where the control character would have caused an action, the sequence is recognized and replaced by the removed character. To transmit the "escape character" itself, two copies are sent. In many
programming language A programming language is a system of notation for writing computer programs. Most programming languages are text-based formal languages, but they may also be graphical. They are a kind of computer language. The description of a programming ...
s and command line interfaces escape sequences are used in
character literal A character literal is a type of literal in programming for the representation of a single character's value within the source code of a computer program. Languages that have a dedicated character data type generally include character literals; ...
s and
string literal A string literal or anonymous string is a string value in the source code of a computer program. Modern programming languages commonly use a quoted sequence of characters, formally " bracketed delimiters", as in x = "foo", where "foo" is a string ...
s, to express characters which are not printable or clash with the syntax of characters or strings. For example, control characters themselves might not be allowed to be placed in the program coded by the editor program, or may have undesirable side-effects if typed into a command. The end-of-quote character is also a problem for programmers that can be solved by escaping it. In most contexts the escape character is the
backslash The backslash is a typographical mark used mainly in computing and mathematics. It is the mirror image of the common slash . It is a relatively recent mark, first documented in the 1930s. History , efforts to identify either the origin of ...
("\").


Samples

For example, the single quotation mark character might be expressed as '\'' since writing is not acceptable. Many modern programming languages specify the doublequote character (") as a delimiter for a string literal. The backslash escape character typically provides ways to include doublequotes inside a string literal, such as by modifying the meaning of the doublequote character embedded in the string (\"), or by modifying the meaning of a sequence of characters including the hexadecimal value of a doublequote character (\x22). Both sequences encode a literal doublequote ("). In
Perl Perl is a family of two high-level, general-purpose, interpreted, dynamic programming languages. "Perl" refers to Perl 5, but from 2000 to 2019 it also referred to its redesigned "sister language", Perl 6, before the latter's name was offic ...
or
Python Python may refer to: Snakes * Pythonidae, a family of nonvenomous snakes found in Africa, Asia, and Australia ** ''Python'' (genus), a genus of Pythonidae found in Africa and Asia * Python (mythology), a mythical serpent Computing * Python (pro ...
2 print "Nancy said "Hello World!" to the crowd."; produces a syntax error, whereas: print "Nancy said \"Hello World!\" to the crowd."; ### example of \" produces the intended output. Another alternative: print "Nancy said \x22Hello World!\x22 to the crowd."; ### example of \x22 uses "\x" to indicate the following two characters are hexadecimal digits, "22" being the ASCII value for a doublequote in hexadecimal. C,
C++ C++ (pronounced "C plus plus") is a high-level general-purpose programming language created by Danish computer scientist Bjarne Stroustrup as an extension of the C programming language, or "C with Classes". The language has expanded significan ...
,
Java Java (; id, Jawa, ; jv, ꦗꦮ; su, ) is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea to the north. With a population of 151.6 million people, Java is the world's mos ...
, and
Ruby A ruby is a pinkish red to blood-red colored gemstone, a variety of the mineral corundum ( aluminium oxide). Ruby is one of the most popular traditional jewelry gems and is very durable. Other varieties of gem-quality corundum are called ...
all allow exactly the same two backslash escape styles. The PostScript language and Microsoft Rich Text Format also use backslash escapes. The
quoted-printable Quoted-Printable, or QP encoding, is a binary-to-text encoding system 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. His ...
encoding uses the
equals sign The equals sign (British English, Unicode) or equal sign (American English), also known as the equality sign, is the mathematical symbol , which is used to indicate equality in some well-defined sense. In an equation, it is placed between tw ...
as an escape character. URL and URI use
percent-encoding Percent-encoding, also known as URL encoding, is a method to encode arbitrary data in a Uniform Resource Identifier (URI) using only the limited US-ASCII characters legal within a URI. Although it is known as ''URL encoding'', it is also used ...
to quote characters with a special meaning, as for non-ASCII characters. Another similar (and partially overlapping) syntactic trick is stropping. Some programming languages also provide other ways to represent special characters in literals, without requiring an escape character (see e.g.
delimiter collision A delimiter is a sequence of one or more characters for specifying the boundary between separate, independent regions in plain text, mathematical expressions or other data streams. An example of a delimiter is the comma character, which acts as ...
).


See also

*
Control character In computing and telecommunication, a control character or non-printing character (NPC) is a code point (a number) in a character set, that does not represent a written symbol. They are used as in-band signaling to cause effects other than the ...
* Escape character *
printf format string The printf format string is a control parameter used by a class of functions in the input/output libraries of C and many other programming languages. The string is written in a simple template language: characters are usually copied literal ...


References

{{DEFAULTSORT:Escape Sequence Control characters