backslash

The backslash ( \ ) is a typographical mark (glyph) used chiefly in computing. It was first introduced to computers in 1960 by Bob Bemer.^[1] Sometimes called a reverse solidus, oblique or a slosh, it is the mirror image of the common slash.^[2]

Other common terms for the character include hack, escape (from C/UNIX), reverse slash, backslant, whack, and backwhack. Also, it is sometimes referred as bash, reverse slant, reversed virgule, or backslat.^[3]

Usage

Bob Bemer introduced the \ character into ASCII, on September 18, 1961, as the result of character frequency studies. In particular the \ was introduced so that the ALGOL boolean operators "∧" (AND) and "∨" (OR) could be composed in ASCII as "/\" and "\/" respectively.^[4] Both these operators were included in early versions of the C programming language supplied with Unix V6 , Unix V7 and more currently BSD 2.11.

In many programming languages such as C and Perl and in Unix scripting languages, the backslash is used to indicate that the character following it should be treated specially. It is sometimes referred to as a knock-down or escape character. In various regular expression languages it acts as a switch, changing literal characters into metacharacters and vice versa. The backslash is used similarly in the TeX typesetting system and in RTF files to begin markup tags. In Haskell, the backslash is used both to introduce special characters and to introduce lambda functions (since it is a reasonable approximation in ASCII of the Greek letter lambda, λ).

In the context of line-oriented text, especially source code for some programming languages, it is often used at the end of a line to indicate that the trailing newline character should be ignored, so that the following line is treated as if it were part of the current line. In this context it may be called a "continuation". The GNU make manual says^[5]

We split each long line into two lines using backslash-newline; this is like using one long line, but is easier to read.

The underlying Windows API can accept either the backslash or slash to separate directory and file components of a path, but the Microsoft convention is to use a backslash, and APIs that return paths put backslash in.^[6] MS-DOS 2.0 copied the hierarchical file system from Unix and thus used the forward slash, but (possibly on the insistence of IBM) added the backslash to allow paths to be typed into the command shell while retaining compatibility with MS-DOS 1.0 and CP/M where the slash was the command-line option indicator. For instance, in a Windows command shell, you can add the "wide" option to the "dir" command by typing "dir/w", yet you can run a program called "w" in a subdirectory "dir" with "dir\w".^[7]

Although the command shell was the only part of MS-DOS that required this, the use of backslash in filenames was propagated to most other parts of the user interface. Today, although the underlying operating system supports either character, some software programs and sub-systems may or may not accept the slash or the backslash as a path delimiter. If slashes are used in path arguments to command-line programs, they may be misinterpreted. Some programs will accept them if the path is placed in double-quotes.^[8] Some built-in security features have failed to understand unexpected-direction slashes in local or internet paths while other parts of the operating system still acted upon them; this has led to some serious lapses in security.^[9][10]

In the Japanese ISO 646 encoding (a 7-bit code based on ASCII), the code point that would be used for backslash in ASCII is instead a yen mark (¥), while in Korean encoding, the code point for backslash is the won currency symbol (₩ or W). Computer programs (such as Windows filenames) still treat it as a backslash in these environments, causing confusion.^[11] Due to extensive use of the backslash code to represent the yen mark in text, some Unicode fonts like MS Mincho render the backslash character as a ¥, so the Unicode characters 00A5 (¥) and 005C (\) look identical when these fonts are selected.

In mathematics, a backslash-like symbol is used for the set difference.

In some dialects of the BASIC programming language, the backslash is used as an operator symbol to indicate integer division.

In MATLAB, the backslash is used for left matrix divide, while the slash is for right matrix divide.

References

External links

• Larry Osterman (2005-06-24), Why is the DOS path character "\"?
• Bob Bemer, How ASCII got its backslash
• Backslash Definition by The Linux Information Project (LINFO)