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In
computer science
Computer science is the study of computation, information, and automation. Computer science spans Theoretical computer science, theoretical disciplines (such as algorithms, theory of computation, and information theory) to Applied science, ...
, 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
Hexadecimal (also known as base-16 or simply hex) is a Numeral system#Positional systems in detail, positional numeral system that represents numbers using a radix (base) of sixteen. Unlike the decimal system representing numbers using ten symbo ...
),
\\, and
\134 (
octal
Octal (base 8) is a numeral system with eight as the base.
In the decimal system, each place is a power of ten. For example:
: \mathbf_ = \mathbf \times 10^1 + \mathbf \times 10^0
In the octal system, each place is a power of eight. For ex ...
) 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 most commonly refers to:
* State (polity), a centralized political organization that regulates law and society within a territory
**Sovereign state, a sovereign polity in international law, commonly referred to as a country
**Nation state, a ...
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 ( ), an acronym for American Standard Code for Information Interchange, is a character encoding standard for representing a particular set of 95 (English language focused) printable character, printable and 33 control character, control c ...
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 a
computer keyboard
A computer keyboard is a built-in or peripheral input device modeled after the typewriter keyboard which uses an arrangement of buttons or Push-button, keys to act as Mechanical keyboard, mechanical levers or Electronic switching system, electro ...
, 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
In engineering, debugging is the process of finding the Root cause analysis, root cause, workarounds, and possible fixes for bug (engineering), bugs.
For software, debugging tactics can involve interactive debugging, control flow analysis, Logf ...
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 ( ), an acronym for American Standard Code for Information Interchange, is a character encoding standard for representing a particular set of 95 (English language focused) printable character, printable and 33 control character, control c ...
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 (''problem solving'', '' mental shortcut'', ''rule of thumb'') is any approach to problem solving that employs a pragmatic method that is not fully optimized, perfected, or rationalized, but is nevertheless ...
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 before ASCII. Each ch ...
.
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 Dale Heatherington and Dennis Hayes for the Hayes Smartmodem in 1981.
The command set consists of a series of short text string ...
, 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 char ...
.
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 character
In computing and telecommunications, an escape character is a character that invokes an alternative interpretation on the following characters in a character sequence. An escape character is a particular case of metacharacters. Generally, the ...
s which change the interpretation of subsequent characters.
ASCII video data terminals
The
VT52
The VT50 is a CRT-based computer terminal that was 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 forwar ...
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 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,
can be used to enable the interpreting of the ANSI (ECMA-48) terminal escape sequences under
DOS
DOS (, ) is a family of disk-based operating systems for IBM PC compatible computers. The DOS family primarily consists of IBM PC DOS and a rebranded version, Microsoft's MS-DOS, both of which were introduced in 1981. Later compatible syste ...
(by using
$e in the
PROMPT command) or in command windows in 16-bit
Windows
Windows is a Product lining, product line of Proprietary software, proprietary graphical user interface, graphical operating systems developed and marketed by Microsoft. It is grouped into families and subfamilies that cater to particular sec ...
. 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
printf is a C standard library function that formats text and writes it to standard output. The function accepts a format c-string argument and a variable number of value arguments that the function serializes per the format string. Mism ...
without resorting to a GUI program.
Use in Linux and Unix displays
The default text terminal, and text windows (such as using
xterm
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 Unix shell, shell. An X display device, dis ...
) respond to ANSI escape sequences.
Quoting escape
Overview
When an
escape character
In computing and telecommunications, an escape character is a character that invokes an alternative interpretation on the following characters in a character sequence. An escape character is a particular case of metacharacters. Generally, the ...
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.
Programming languages are described in terms of their Syntax (programming languages), syntax (form) and semantics (computer science), semantics (meaning), usually def ...
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
string literal or anonymous string is a literal for 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 ...
s, to express characters which are not printable or clash with the syntax of characters or strings. For example,
control characters
In computing and telecommunications, a control character or non-printing character (NPC) is a code point in a character set that does not represent a written character or symbol. They are used as in-band signaling to cause effects other than ...
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 mark used mainly in computing and mathematics. It is the mirror image of the common slash (punctuation), slash . It is a relatively recent mark, first documented in the 1930s. It is sometimes called a hack, whack, Escape c ...
("\").
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
A delimiter is a sequence of one or more Character (computing), characters for specifying the boundary between separate, independent regions in plain text, Expression (mathematics), mathematical expressions or other Data stream, data streams. An ...
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 high-level, general-purpose, interpreted, dynamic programming language. Though Perl is not officially an acronym, there are various backronyms in use, including "Practical Extraction and Reporting Language".
Perl was developed ...
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 is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea (a part of Pacific Ocean) to the north. With a population of 156.9 million people (including Madura) in mid 2024, proje ...
, and
Ruby
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 sapph ...
all allow exactly the same two backslash escape styles. The
PostScript
PostScript (PS) is a page description language and dynamically typed, stack-based programming language. It is most commonly used in the electronic publishing and desktop publishing realm, but as a Turing complete programming language, it c ...
language and Microsoft
Rich Text Format
)
As an example, the following RTF code
would be rendered as follows:
This is some bold text.
Character encoding
A standard RTF file can only consist of 7-bit ASCII characters, but can use escape sequences to encode other characters. ...
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. Hi ...
encoding uses the
equals sign
The equals sign (British English) or equal sign (American English), also known as the equality sign, is the mathematical symbol , which is used to indicate equality. In an equation it is placed between two expressions that have the same valu ...
as an escape character.
URL
A uniform resource locator (URL), colloquially known as an address on the Web, is a reference to a resource that specifies its location on a computer network and a mechanism for retrieving it. A URL is a specific type of Uniform Resource Identi ...
and
URI
Uri may refer to:
Places
* Canton of Uri, a canton in Switzerland
* Úri, a village and commune in Hungary
* Uri, Iran, a village in East Azerbaijan Province
* Uri, Jammu and Kashmir, a town in India
* Uri (island), off Malakula Island in V ...
use
percent-encoding
URL encoding, officially known as percent-encoding, is a method to binary-to-text encoding, encode arbitrary data in a uniform resource identifier (URI) using only the ASCII, US-ASCII characters legal within a URI. Although it is known as ''URL en ...
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 ...
).
See also
*
Control character
In computing and telecommunications, a control character or non-printing character (NPC) is a code point in a character encoding, character set that does not represent a written Character (computing), character or symbol. They are used as in-ba ...
*
Escape character
In computing and telecommunications, an escape character is a character that invokes an alternative interpretation on the following characters in a character sequence. An escape character is a particular case of metacharacters. Generally, the ...
*
printf format string
printf is a C standard library function that formats text and writes it to standard output. The function accepts a format c-string argument and a variable number of value arguments that the function serializes per the format string. Mism ...
*
format control string
References
{{DEFAULTSORT:Escape Sequence
Control characters