The backslash (\) is a typographical mark (glyph) used mainly in computing and is the mirror image of the common slash. It is sometimes called a reverse solidus, slosh, hack, escape (from C/UNIX), reverse slash, backslant, backwhack, bash, reverse slant, reversed virgule, and backslat. In Unicode, it is encoded at U+005C \ reverse solidus (HTML: \
).
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. 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 (if it would otherwise be treated normally), or normally (if it would otherwise 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).