Escape sequence

• In C and many derivative programming languages, a string escape sequence is a series of two or more characters, starting with a backslash \.[3]
  • 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.[4]
  • 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.[5]
  • A character may be escaped in multiple different ways. Assuming ASCII encoding, the escape sequences \x5c (hexadecimal), \\, \134 (octal) 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 [ (decimal character code 91) defines an escape sequence.

When directed this series of characters is used to change the state of computers and their attached peripheral devices, rather than to be displayed or printed as regular 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 "[" or a specially-allocated CSI character with a code 155 (decimal).

Not all control sequences used an escape character; for example:

• modem control sequences used by AT/Hayes-compatible modems[6][7]

• Data General terminal control sequences,[8][9][10] 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).[11][12] They were common when most dumb terminals used ASCII 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.

An escape character is usually assigned to the Esc key on a computer keyboard, 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,[13] or for backing up one level in a menu in some applications.[14] The Hewlett Packard HP 2640 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 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 can fail under some circumstances, especially without fast modern communication speeds.

Escape sequences date back at least to the 1874 Baudot code.[15][16][17]

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.[18][19] 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.

A control character is a character that, in isolation, has some control function, such as carriage return (CR). Escape sequences, by contrast, consist of one or more escape characters which change the interpretation of subsequent characters.

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 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.

• Control character
• Escape character
• printf format string
• format control string