On Unix systems, and in many programming languages such as C and Perl, 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 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
We split each long line into two lines using backslash-newline; this is like using one long line, but is easier to read.In DOS and Microsoft Windows, either the backslash or slash can be used as the delimiter between directories and filenames in path expressions. This is in contrast to Unix paths and Internet URLs (web addresses), which only use the forward slash. In an early version of DOS, which did not support directories and thus had no need for a path delimiter, the forward slash was used to introduce command-line options (in Unix, the hyphen ["-"] is used for this purpose.) When directories were introduced to DOS, another character had to be chosen to be able to represent the delimiter, and the backslash was selected.
The backslash's prominence in Microsoft path names might explain why the forward slashes in URLs are occasionally (and erroneously) read out loud as "backslash". It has even led to its erroneous placement in contexts not relating to directories, or computers at all, for that matter. URLs always exclusively contain slashes, sometimes referred to as "forward" slashes in an attempt to clarify the distinction.
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 on Korean computer keyboards, the backslash corresponds to the won symbol (₩ or
W). Many Japanese environments nonetheless treat it like a backslash, causing confusion. To add to the confusion, some fonts, like MS Mincho, render the backslash character as a ¥, so the Unicode characters 00A5 (¥) and 005C () look identical when these fonts are selected.