Escape character

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In computing and telecommunication, an escape character is a character which invokes an alternative interpretation on subsequent characters in a character sequence. An escape character is a particular case of metacharacters. Generally, the judgement of whether something is an escape character or not depends on context.

Definition

Escape characters are part of the syntax for many programming languages, data formats, and communication protocols. For a given alphabet an escape character's purpose is to start character sequences (so named escape sequences), which have to be interpreted differently from the same characters occurring without the prefixed escape character. An escape character may not have its own meaning, so all escape sequences are of two or more characters.

There are usually two functions of escape sequences. The first is to encode a syntactic entity, such as device commands or special data, which cannot be directly represented by the alphabet. The second use, referred to as character quoting, is to represent characters, which cannot be typed in current context, or would have an undesired interpretation. In the latter case an escape sequence is a digraph consisting of an escape character itself and a "quoted" character.

Control character

Generally, an escape character is not a particular case of (device) control characters, nor vice versa. If we define control characters as non-graphic, or as having a special meaning for an output device (e.g. printer or text terminal) then any escape character for this device is a control one. But escape characters used in programming (such as the backslash, "\") are graphic, hence are not control characters. Conversely most (but not all) of the ASCII "control characters" have some control function in isolation, therefore are not escape characters.

In many programming languages, an escape character also forms some escape sequences which are referred to control characters. For example, line break has an escape sequence of \n.

Examples

JavaScript

JavaScript uses the \ (backslash) as an escape characters for:[1][2]

  • \' single quote
  • \" double quote
  • \\ backslash
  • \n new line
  • \r carriage return
  • \t tab
  • \b backspace
  • \f form feed
  • \v vertical tab (IE < 9 treats '\v' as 'v' instead of a vertical tab ('\x0B'). If cross-browser compatibility is a concern, use \x0B instead of \v.)
  • \0 null character (U+0000 NULL) (only if the next character is not a decimal digit; else it is an octal escape sequence)

Note that the \v and \0 escapes are not allowed in JSON strings.

ASCII escape character

The ASCII "escape" character (octal: \033, hexadecimal: \x1B, or ^[, or, in decimal, 27) is used in many output devices to start a series of characters called a control sequence or escape sequence. Typically, the escape character was sent first in such a sequence to alert the device that the following characters were to be interpreted as a control sequence rather than as plain characters, then one or more characters would follow to specify some detailed action, after which the device would go back to interpreting characters normally. For example, the sequence of ^[, followed by the printable characters [2;10H, would cause a DEC VT102 terminal to move its cursor to the 10th cell of the 2nd line of the screen. This was later developed to ANSI escape codes covered by the ANSI X3.64 standard. The escape character also starts each command sequence in the Hewlett Packard Printer Command Language.

Early reference to the term "escape character" is found in Bob Bemer's IBM technical publications. Apparently, it is he who invented this mechanism, during his work on the ASCII character set.

The Escape key is usually found on standard PC keyboards. However it is commonly absent from keyboards for PDAs and other devices not designed primarily for ASCII communications. The DEC VT220 series was one of the few popular keyboards that did not have a dedicated Esc key, instead using one of the keys above the main keypad. In user interfaces of the 1970s–1980s it was not uncommon to use this key as an escape character, but in modern desktop computers such use is dropped. Sometimes the key was identified with AltMode (for alternative mode). Even with no dedicated key, the escape character code could be generated by typing '[' while simultaneously holding down the Control key, 'Ctrl'.

Programming and data formats

Many modern programming languages specify the doublequote character (") as a delimiter for a string literal. The backslash (\) escape character typically provides two ways to include doublequotes inside a string literal, either by modifying the meaning of the doublequote character embedded in the string (\" becomes "), or by modifying the meaning of a sequence of characters including the hexadecimal value of a doublequote character (\x22 becomes ").

C, C++, Java, and Ruby all allow exactly the same two backslash escape styles. The PostScript language and Microsoft Rich Text Format also use backslash escapes. The quoted-printable encoding uses the equals sign as an escape character.

URL and URI use %-escapes to quote characters with a special meaning, as for non-ASCII characters. The ampersand (&) character may be considered as an escape character in SGML and derived formats such as HTML and XML.

Some programming languages also provide other ways to represent special characters in literals, without requiring an escape character (see e.g. delimiter collision).

Communication protocols

The Point-to-Point Protocol uses the 0x7D octet (\175, or ASCII: } ) as an escape character. The octet immediately following should be XORed by 0x20 before being passed to a higher level protocol. This is applied to both 0x7D itself and the control character 0x7E (which is used in PPP to mark the beginning and end of a frame) when those octets need to be transmitted by a higher level protocol encapsulated by PPP, as well as other octets negotiated when the link is established. That is, when a higher level protocol wishes to transmit 0x7D, it is transmitted as the sequence 0x7D 0x5D, and 0x7E is transmitted as 0x7D 0x5E.

Bourne shell

In Bourne shell (sh), the asterisk (*) and question mark (?) characters are wildcard characters expanded via globbing. Without a preceding escape character, an * will expand to the names of all files in the working directory that do not start with a period iff there are such files, otherwise * remains unexpanded. So to refer to a file literally called "*", the shell must be told not to interpret it in this way, by preceding it with a backslash (\). This modifies the interpretation of the asterisk (*). Compare:

  
rm *    # delete all files in the current directory

rm \*   # delete the file named *

Windows Command Prompt

The Windows command-line interpreter uses a caret character (^) to escape reserved characters that have special meanings (in particular: & | ( ) < > ^).[3] The DOS command-line interpreter, though it supports similar syntax, does not support this.

For example, on the Windows Command Prompt, this will result in a syntax error.

echo <wiki>

whereas this will output the string: <wiki>

echo ^<wiki^>

Others

See also

Not to be confused with:

References

  1. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  2. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  3. ↑ Lua error in package.lua at line 80: module 'strict' not found.

External links

 This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C".

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