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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 (includin ...
, an escape sequence is a combination of
characters 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
In mathematics and computing, the hexadecimal (also base-16 or simply hex) numeral system is a positional numeral system that represents numbers using a radix (base) of 16. Unlike the decimal system representing numbers using 10 symbols, h ...
),
\\,
\134 (
octal
The octal numeral system, or oct for short, is the radix, base-8 number system, and uses the Numerical digit, digits 0 to 7. This is to say that 10octal represents eight and 100octal represents sixty-four. However, English, like most languages, ...
) 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 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 ...
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
keycaps.
With the introduction of ANSI terminals most escape sequences began with the ''two'' characters "ESC" then "
Hayes-compatible modems
* Data General">Hayes command set">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).
They were common when most dumb terminals 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 esca ...
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 technology ...
, 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 ...
terminals had a key for a "display functions" mode which would display graphics for all control characters, including Esc, to aid in
debugging
In computer programming and software development, debugging is the process of finding and resolving ''bugs'' (defects or problems that prevent correct operation) within computer programs, software, or systems.
Debugging tactics can involve in ...
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 immediat ...
can fail under some circumstances, especially without fast modern communication speeds.
Escape sequences date back at least to the 1874
Baudot code.
Modem control
The
Hayes command set, 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, 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 ch ...
.
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 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 terminal implemented the more sophisticated
ANSI escape sequences 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 ...
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,
can be used to enable the interpreting of the ANSI (ECMA-48) terminal escape sequences under
DOS (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 ...
. The rise of
GUI 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 liter ...
without resorting to a GUI program.
Use in Linux and Unix displays
The default text terminal, and text windows (such as using
xterm) 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 l ...
s and command line interfaces escape sequences are used in
character literals and
string literals, 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 ("\").
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 programming language, high-level, General-purpose programming language, general-purpose, Interpreter (computing), interpreted, dynamic programming languages. "Perl" refers to Perl 5, but from 2000 to 2019 it ...
or
Python 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++,
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 mo ...
, 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 sapp ...
all allow exactly the same two backslash escape styles. The
PostScript
PostScript (PS) is a page description language in the electronic publishing and desktop publishing realm. It is a dynamically typed, concatenative programming language. It was created at Adobe Systems by John Warnock, Charles Geschke, ...
language and Microsoft
Rich Text Format also use backslash escapes. The
quoted-printable 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 ...
as an escape character.
URL
A Uniform Resource Locator (URL), colloquially termed as a web address, is a reference to a web resource that specifies its location on a computer network and a mechanism for retrieving it. A URL is a specific type of Uniform Resource Identifie ...
and
URI use
percent-encoding 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
*
Escape character
*
printf format string
References
{{DEFAULTSORT:Escape Sequence
Control characters