View source for Backslash

{{Short description|Typographical mark (\)}}
{{Distinguish|Backlash (disambiguation){{!}}Backlash|₩|¥}}
{{Technical reasons|:\|the keyboard symbol|List of emoticons}}
{{Infobox symbol 
|mark=\
|unicode={{unichar|005C|REVERSE SOLIDUS|html=}}
|see also={{unichar|29F5|Reverse solidus operator}}<br />
{{unichar| 29F9 |Big reverse solidus}}<br />
{{unichar| FE68 |nlink=Small Form Variants}}<br />
{{unichar|FF3C |Fullwidth reverse solidus}}
}}

The '''backslash''' {{char|'''\'''}} is a  mark used mainly in [[computing]] and [[mathematics]]. It is the [[mirror image]] of the common [[slash (punctuation)|slash]] {{char|/}}.  It is a relatively recent mark, first documented in the 1930s. It is sometimes called a '''hack''', '''whack''', '''[[Escape character|escape]]''' (from [[C (programming language)|C]]/[[UNIX]]), '''reverse slash''', '''slosh''', '''downwhack''', '''backslant''', '''backwhack''', '''bash''', '''reverse slant''', '''reverse solidus''', and '''reversed virgule'''.<ref>''[[Macquarie Dictionary]]'' (3rd edition)</ref><ref>{{cite web | url=http://www.catb.org/jargon/html/A/ASCII.html | title=ASCII | last=Raymond | first=Eric S | website=The Jargon File | access-date=2007-07-28 | archive-date=2019-01-18 | archive-url=https://web.archive.org/web/20190118060854/http://catb.org/jargon/html/A/ASCII.html | url-status=live }}</ref>

==History==
<!-- The copyright on Teletype Corporation documents cited in this section (and held in an independent third party repository) has expired. -->
[[File:Teletype Wheatstone Perforator keyboard.jpg|thumb|A Teletype Wheatstone Perforator keyboard from the 1930s, with backslash in the end of the third row]]
[[File:Mappa Teletype ASR-33.jpg|thumb|Teletype ASR-33 keyboard layout with ASCII character set, prior to June 14, 1966, with backslash on shift+L]]
{{as of|2022|11}}, efforts to identify either the origin of this character or its purpose before the 1960s have not been successful. The earliest known reference found to date is a 1937 maintenance manual from the [[Teletype Corporation]] with a photograph showing the keyboard of its [[Kleinschmidt keyboard perforator]] WPE-3 using the [[Wheatstone system]].<ref>{{citation |mode=cs1 |title=Bulletin 125, issue 2: Description and Adjustments of the Teletype Wheatstone Perforator |publisher=Teletype Corporation |date=May 1938 |orig-date=August 1937 |url=http://www.navy-radio.com/manuals/tty/tty125.pdf |page=ii |via=navy-radio.com |access-date=2021-03-15 |archive-date=2021-02-12 |archive-url=https://web.archive.org/web/20210212121807/http://www.navy-radio.com/manuals/tty/tty125.pdf |url-status=live }}</ref><ref name=Fischer /> The symbol was called the "diagonal key",<ref>{{cite web |title=Bulletin 188: Teletype automatic perforator set |page=6 |url=https://www.navy-radio.com/manuals/tty/188B-4508.pdf |publisher=Teletype Corporation |date=August 1945 |via=navy-radio.com |access-date=2021-12-19 |archive-date=2021-12-19 |archive-url=https://web.archive.org/web/20211219004736/https://www.navy-radio.com/manuals/tty/188B-4508.pdf |url-status=live}}</ref> and given a (non-standard) [[Morse code]] of {{morse|dot|dash|dot|dot|dash}}.<ref>{{cite web |title=Bulletin 1025, issue 3: Parts for Teletype Wheatstone Perforator |page=9 |url=https://www.navy-radio.com/manuals/tty/tty1025.pdf |date=July 1945 |publisher=Teletype Corporation |via=navy-radio.com |access-date=2021-12-19 |archive-date=2021-12-19 |archive-url=https://web.archive.org/web/20211219003141/https://www.navy-radio.com/manuals/tty/tty1025.pdf |url-status=live }}</ref>{{efn|This is the inverse of the code for the slash symbol ({{morse|dash|dot|dot|dash|dot}}).}}

In June 1960, [[IBM]] published an "Extended character set standard" that includes the symbol at 0x19.<ref name=Fischer>{{cite web |title=The Evolution of Character Codes, 1874–1968 |date=20 June 2000 |first=Eric |last=Fischer |pages=14–15 (Table 35) |url=http://dump.deadcodersociety.org/ascii.pdf |access-date=30 November 2022 |archive-date=30 November 2022 |archive-url=https://web.archive.org/web/20221130065133/http://dump.deadcodersociety.org/ascii.pdf |url-status=live }} Referencing {{cite tech report |first1=R.W. |last1=Bemer |first2=W. |last2=Buchholz |title=An Extended Character Set Standard |id=TR 00.721  |publisher=Product Development Laboratory, Data Systems Division, International Business Machines Corporation |date=June 1, 1960}} Computer Standards Collection, Archives Center, National Museum of American History, Smithsonian Institution, box 1.</ref> In September 1961, [[Bob Bemer]] (IBM) proposed to the [[International Committee for Information Technology Standards|X3.2 standards committee]] that {{code|[}}, {{code|]}} and {{code|\}} be made part of the proposed standard, describing the backslash as a "reverse division operator" and cited its prior use by Teletype in telecommunications. In particular, he said, the {{mono|\}} was needed so that the [[ALGOL 68#Standard dyadic operators with associated priorities|ALGOL Boolean operators]] {{code|∧}} ([[logical conjunction]]) and {{code|∨}} ([[logical disjunction]]) could be composed using {{mono|/\}} and {{mono|\/}} respectively. The Committee adopted these changes into the draft American Standard (subsequently called [[ASCII]]) at its November 1961 meeting.<ref name=Fischer />

These operators were used for min and max in early versions of the [[C (programming language)|C programming language]] supplied with Unix V6<ref>{{cite web |url=http://www.bitsavers.org/bits/Interdata/32bit/unix/univWollongong%5Fv6/interdata%5Fv6/usr/source/c/c00.c |title=C compiler source |date=1975 |website=bitsavers.org |access-date=2009-10-11 |archive-date=2011-08-15 |archive-url=https://web.archive.org/web/20110815153104/http://www.bitsavers.org/bits/Interdata/32bit/unix/univWollongong_v6/interdata_v6/usr/source/c/c00.c |url-status=live }}</ref> and V7.<ref>{{cite web |url=http://stuff.mit.edu/afs/sipb/project/v7unix/fs/usr/src/cmd/c/c00.c |title=C compiler source |date=1979 |website=mit.edu |access-date=2009-10-11 |archive-date=2013-12-03 |archive-url=https://web.archive.org/web/20131203022053/http://stuff.mit.edu/afs/sipb/project/v7unix/fs/usr/src/cmd/c/c00.c |url-status=live }}</ref>

==Usage==

===Programming languages===
In many [[programming language]]s such as [[C (programming language)|C]], [[Perl]], [[PHP]], [[Python (programming language)|Python]], [[Unix]] scripting languages, and many file formats such as [[JSON]], the backslash is used as an [[escape character]], to indicate that the character following it should be treated specially (if it would otherwise be treated literally), or literally (if it would otherwise be treated specially). For instance, inside a C [[string literal]] the sequence {{code|\n}} produces a [[newline]] byte instead of an 'n', and the sequence {{code|\"}} produces an actual double quote rather than the special meaning of the double quote ending the string. An actual backslash is produced by a double backslash {{code|\\}}.

[[Regular expression]] languages used it the same way, changing subsequent literal characters into [[metacharacter]]s and vice versa. For instance {{mono|\{{vbar}}{{vbar}}b}} searches for either '|'  or 'b', the first bar is escaped and searched for, the second is not escaped and acts as an "or".

{{anchor|line-continuation}}Outside quoted strings, the only common use of backslash is to ignore ("escape") a newline immediately after it. In this context it may be called a "continued line"<ref>{{cite web |url=https://gcc.gnu.org/onlinedocs/gcc-3.2.3/cpp/Initial-processing.html |title=The C Preprocessor |website=[[GNU]].org |access-date=2021-01-04 |archive-date=2021-08-03 |archive-url=https://web.archive.org/web/20210803000553/https://gcc.gnu.org/onlinedocs/gcc-3.2.3/cpp/Initial-processing.html |url-status=live }}</ref> as the current line continues into the next one. Some software replaces the backslash+newline with a space.<ref>{{cite book|chapter=3.1.1 Splitting long lines|title=GNU {{font|make|size=80%}} manual|chapter-url=https://www.gnu.org/software/make/manual/make.html#Splitting-Lines|access-date=July 28, 2019|archive-date=June 5, 2018|archive-url=https://web.archive.org/web/20180605023853/https://www.gnu.org/software/make/manual/make.html#Splitting-Lines|url-status=live}}</ref>

To support [[computers]] that lacked the backslash character, the [[C trigraph]] {{code|??/}} was added, which is equivalent to a backslash. Since this can escape the next character, which may itself be a {{code|?}}, the primary modern use may be for [[code obfuscation]]. Support for trigraphs in [[C++]] was removed in [[C++17]], and support for them in C is planned to be removed in [[C23 (C standard revision)|C23]].

In [[Visual Basic]] (and some other [[BASIC]] dialects) the backslash is used as an operator symbol to indicate [[Integer (computer science)|integer]] [[Division (mathematics)|division]].<ref>{{cite web|title=Arithmetic Operators in Visual Basic|url=http://msdn.microsoft.com/en-us/library/b6ex274z(v=vs.110).aspx|work=Visual Basic Language Features: Operators and Expressions| date=10 July 2012 |publisher=MSDN|access-date=7 October 2012|archive-date=17 September 2012|archive-url=https://web.archive.org/web/20120917100112/http://msdn.microsoft.com/en-us/library/b6ex274z(v=vs.110).aspx|url-status=live}}</ref> This rounds toward zero.

The [[ALGOL 68]] programming language uses the "\" as its [[Scientific notation|Decimal Exponent Symbol]]. ALGOL 68 has the choice of 4 Decimal Exponent Symbols: e, E, \, or <sub>10</sub>.  Examples: {{mono|6.0221415e23}}, {{mono|6.0221415E23}}, {{mono|6.0221415\23}} or {{mono|6.0221415<sub>10</sub>23}}.<ref>{{cite journal | title=Revised Report on the Algorithmic Language Algol 68 | journal=Acta Informatica | volume=5 | issue=1–3 | pages=1–236 | date=September 1973 | doi=10.1007/BF00265077 | s2cid=2490556 | url=https://ir.cwi.nl/pub/13447 | access-date=2020-02-10 | archive-date=2020-07-26 | archive-url=https://web.archive.org/web/20200726125450/https://ir.cwi.nl/pub/13447 | url-status=live }}</ref>

In [[APL (programming language)|APL]] {{mono|\}} is called ''Expand'' when used to insert fill elements into arrays, and ''Scan'' when used to produce prefix reduction (cumulative fold).

In [[PHP]] version 5.3 and higher, the backslash is used to indicate a [[namespace]].<ref>{{cite web |url=http://php.net/manual/en/language.namespaces.rationale.php |title=Namespaces overview |website=php.net |access-date=2012-10-10 |archive-date=2012-10-16 |archive-url=https://web.archive.org/web/20121016184321/http://www.php.net/manual/en/language.namespaces.rationale.php |url-status=live }}</ref>

In [[Haskell (programming language)|Haskell]], the backslash is used both to introduce special characters and to introduce [[anonymous function|lambda functions]] (since it is a reasonable approximation in ASCII of the Greek letter {{nowrap|lambda, λ)}}.<ref>O'Sullivan, Stewart, and Goerzen,  ''Real World Haskell'', ch. 4: anonymous (lambda) functions, p.99</ref>

===Filenames===

[[MS-DOS]] 2.0, released 1983, copied the idea of a [[hierarchical file system]] from [[Unix]] and thus used the (forward) [[slash (punctuation)|slash]] as the directory separator.<ref>{{cite web |url=https://learn.microsoft.com/en-us/archive/blogs/larryosterman/why-is-the-dos-path-character |title=Why is the DOS path character "\"? |publisher=Blogs.msdn.com |date=2005-06-24 |access-date=2013-06-16 |archive-date=2022-11-19 |archive-url=https://web.archive.org/web/20221119103949/https://learn.microsoft.com/en-us/archive/blogs/larryosterman/why-is-the-dos-path-character |url-status=live }}</ref> Possibly on the insistence of IBM,<ref>{{ cite web | url = http://www.os2museum.com/wp/why-does-windows-really-use-backslash-as-path-separator/ | title = Why Does Windows Really Use Backslash as Path Separator? | access-date = 28 May 2019 | last = Necasek | first = Michal | date = 24 May 2019 | website = <nowiki>OS/2 Museum</nowiki> | archive-url = https://web.archive.org/web/20190524202002/http://www.os2museum.com/wp/why-does-windows-really-use-backslash-as-path-separator/ | archive-date = 24 May 2019 | df = dmy-all }}</ref><ref>{{Citation |title=MS-DOS v1.25 and v2.0 Source Code, README.txt |date=1983<!-- https://github.com/microsoft/MS-DOS/blob/04a3d20ff411409ab98474892b2bb1713bde0f7f/v2.0/source/UINIT.ASM#L22&:~:text=Corp.%201981%2C%201982-,%2C%201983,-%22%2C13 --> |first1=Chris |last1=Larson <!-- MS-DOS Product Marketing Manager --> |first2=Don |last2=Immerwahr <!-- Microsoft OEM technical support--> |url=https://github.com/microsoft/MS-DOS/blob/master/v2.0/source/README.txt#L41&:~:text=last%20minute%20changes,the%20switch%20character. |publisher=Microsoft |access-date=21 November 2022 |quote=The user manual contains some significant errors.  Most of these are due to last minute changes to achieve a greater degree of compatibility with IBM's implementation of MS-DOS (PC DOS).  This includes the use of "\" instead of "/" as the path separator, and "/" instead of "-" as the switch character.  For transporting of batch files across machines, Microsoft encourages the use of "\" and "/" respectively in the U.S. market.  [...] We apologize for any inconveniences these changes may have caused your technical publications staff. |archive-date=21 November 2022 |archive-url=https://web.archive.org/web/20221121055228/https://github.com/microsoft/MS-DOS/blob/master/v2.0/source/README.txt#L41&:~:text=last%20minute%20changes,the%20switch%20character. |url-status=live }}</ref> Microsoft added the backslash to allow paths to be typed at the command line interpreter prompt, while retaining compatibility with MS-DOS 1.0 (in which {{char|/}} was the command-line option indicator. Typing "{{mono|DIR/W}}" gave the "wide" option to the "{{mono|[[DIR (command)|DIR]]}}" command, so some other method was needed if one actually wanted to run a program called {{mono|W}} inside a directory called {{mono|DIR}}). Except for [[COMMAND.COM]], all other parts of the operating system accept both characters in a [[path (computing)|path]], but the Microsoft convention remains to use a backslash, and [[Application programming interface|APIs]] that ''return'' paths use backslashes.<ref>
{{cite web
 | url = http://msdn.microsoft.com/en-us/library/system.io.path.getfullpath.aspx
 | title = Path.GetFullPath Method
 | work = .NET Framework Class Library
 | publisher = Microsoft
 | access-date = 2009-01-02
| archive-url= https://web.archive.org/web/20081221114738/http://msdn.microsoft.com/en-us/library/system.io.path.getfullpath.aspx| archive-date= 21 December 2008 | url-status= live}}</ref> In some versions<!-- MS-DOS/PC DOS 2.xx-3.xx, all versions of DR-DOS since 5.0, as well as PTS-DOS and FreeDOS, but not in FlexOS, 4680 OS, 4690 OS. -->, the option character can be changed from {{mono|/}} to {{mono|-}} via [[SWITCHAR]], which allows COMMAND.COM to preserve {{mono|/}} in the command name.

The [[Microsoft Windows]] family of operating systems inherited the MS-DOS behavior and so still support either character – but individual Windows programs and sub-systems may, wrongly, only accept the backslash as a path delimiter, or may misinterpret a forward slash if it is used as such. Some programs will only accept forward slashes if the path is placed in [[Quotation mark|double-quotes]].<ref>
{{cite web
 | url = http://bytes.com/groups/python/23123-when-did-windows-start-accepting-forward-slash-path-separator
 | title = When did Windows start accepting forward slash as a path separator?
 | publisher = Bytes.com
 | access-date = 2009-01-02
| archive-url= https://web.archive.org/web/20090206050805/http://bytes.com/groups/python/23123-when-did-windows-start-accepting-forward-slash-path-separator| archive-date= 6 February 2009 | url-status= live}}</ref> The failure of Microsoft's security features to recognize unexpected-direction slashes in local and Internet paths, while other parts of the operating system still act upon them, has led to some serious lapses in security. Resources that should not be available have been accessed with paths using particular mixes, such as {{mono|<nowiki>http://example.net/secure\private.aspx</nowiki>}}.<ref>{{cite web
 |url=http://www.devsource.com/c/a/Using-VS/Microsoft-Probes-Flaw-in-ASPNET/
 |archive-url=https://archive.today/20130121115529/http://www.devsource.com/c/a/Using-VS/Microsoft-Probes-Flaw-in-ASPNET/
 |url-status=dead
 |archive-date=2013-01-21
 |title=Microsoft Probes Flaw in ASP.NET 
 |last=Kaplan 
 |first=Simone 
 |publisher=Ziff Davis Enterprise Holdings Inc. 
 |work=DevSource, sponsored by Microsoft 
 |year=2004 
 |access-date=2009-06-14 
}}</ref><ref>{{cite web
| url=http://www.securityfocus.com/columnists/285
| title=Security Holes That Run Deep
| last=Burnett
| first=Mark
| publisher=SecurityFocus
| year=2004
| access-date=2009-06-14
| archive-date=2021-02-02
| archive-url=https://web.archive.org/web/20210202211420/https://www.securityfocus.com/columnists/285
| url-status=live
}}</ref>

===Text markup===

The backslash is used in the [[TeX]] [[typesetting]] system and in [[Rich Text Format|RTF]] files to begin markup tags.

In [[Unified Standard Format Markers|USFM]],<ref>{{cite web|url=http://paratext.org/usfm|title=USFM – Unified Standard Format Markers|work=paratext.org|date=18 April 2017|access-date=30 June 2013|archive-date=13 August 2020|archive-url=https://web.archive.org/web/20200813060533/https://paratext.org/usfm/|url-status=live}}</ref> the backslash is used to mark format features for editing [[Bible translations]].

In [[caret notation]], {{code|^\}} represents the control character 0x1C, [[file separator]].<ref>{{cite web    |title=C0 control characters set, 1987    |url=http://wwwcdf.pd.infn.it/AppuntiLinux/a2138.htm#almlanchor1706    |website=pd.infn.it    |access-date=2021-03-14    |archive-date=2019-09-06    |archive-url=https://web.archive.org/web/20190906020303/http://wwwcdf.pd.infn.it/AppuntiLinux/a2138.htm#almlanchor1706    |url-status=live    }}</ref> This is entirely a coincidence and has nothing to do with its use in file paths.

===Mathematics===

A backslash-like symbol is used for the [[complement (set theory)|set difference]].<ref>{{cite web|title=Quantities and units – Part 2: Mathematical signs and symbols to be used in the natural sciences and technology|url=http://www.iso.org/iso/catalogue_detail?csnumber=31887|work=ISO 80000-2:2009| date=15 April 2008 |publisher=International Organization for Standardization|access-date=2012-10-07|archive-date=2014-03-26|archive-url=https://web.archive.org/web/20140326013017/http://www.iso.org/iso/catalogue_detail?csnumber=31887|url-status=live}}</ref>

The backslash is also sometimes used to denote the right [[coset]] space.<ref>{{cite web |title=Definition:Coset Space |url=https://proofwiki.org/w/index.php?title=Definition:Coset_Space&oldid=227233 |publisher=ProofWiki |access-date=1 February 2017 |archive-date=26 February 2021 |archive-url=https://web.archive.org/web/20210226163838/https://proofwiki.org/w/index.php?title=Definition:Coset_Space&oldid=227233 |url-status=live }}</ref>

Especially when describing computer algorithms, it is common to define backslash so that {{math|''a''\''b''}} is equivalent to {{math|{{floor|''a''/''b''}}}}.<ref>{{cite web|title=Integer Division|url=https://mathworld.wolfram.com/IntegerDivision.html|website=mathworld.wolfram.com|access-date=2020-03-10|archive-date=2020-02-21|archive-url=https://web.archive.org/web/20200221154259/http://mathworld.wolfram.com/IntegerDivision.html|url-status=live}}</ref> This is integer division that rounds down, not towards zero.

In [[MATLAB]] and [[GNU Octave]] the backslash is used for left [[Matrix division|matrix divide]], while the (forward) slash is for right matrix divide.<ref>{{cite web|last=Eaton|first=John W.|title=GNU Octave: A high-level interactive language for numerical computations|url=https://www.gnu.org/software/octave/octave.pdf|publisher=Free Software Foundation|access-date=7 October 2012|author2=David Bateman|author3=Søren Hauberg|page=145|date=February 2011|archive-date=24 October 2012|archive-url=https://web.archive.org/web/20121024061422/http://www.gnu.org/software/octave/octave.pdf|url-status=live}}</ref>

==Confusion with ¥ and other characters==

In the [[Japanese language|Japanese]] encodings [[JISCII|ISO 646-JP]] (a 7-bit code based on [[ASCII]]), [[JIS X 0201]] (an 8-bit code), and [[Shift JIS]] (a multi-byte encoding which is 8-bit for ASCII), the [[code point]] 0x5C that would be used for backslash in ASCII is instead rendered as a [[yen sign]] {{char|¥}}. Due to extensive use of the 005C code point to represent the yen sign, even today some fonts such as [[MS Mincho]] render the backslash character as a ¥, so the characters at Unicode [[code point]]s 00A5 (¥) and 005C (\) both render as {{char|¥}} when these fonts are selected. Computer programs still treat 005C as a backslash in these environments but display it as a yen sign, causing confusion, especially in MS-DOS filenames.<ref>{{cite web |url=http://www.siao2.com/2005/09/17/469941.aspx |title=When is a backslash not a backslash? |publisher=Blogs.msdn.com |access-date=2013-06-16 |archive-date=2016-03-23 |archive-url=https://web.archive.org/web/20160323062003/http://www.siao2.com/2005/09/17/469941.aspx |url-status=live }}</ref>

Several other [[ISO 646]] versions also replace backslash with other characters, including [[₩]] (Korean), [[Ö]] (German, Swedish), [[Ø]] (Danish, Norwegian), [[ç]] (French) and [[Ñ]] (Spanish), leading to similar problems, though with less lasting impact compared to the yen sign.

In 1991, RFC 1345 suggested {{code|//}} as a unique two-character [[mnemonic]] that might be used in internet standards as "a practical way of identifying [this] character, without reference to a coded character set and its code in [that] coded character set".<ref name="RFC1345">{{cite web |url=https://tools.ietf.org/html/rfc1345 |title=RFC 1345 |publisher=Tools.ietf.org |date=1991-12-26 |access-date=2013-06-16 |last1=Simonsen |first1=Keld |archive-date=2016-06-13 |archive-url=https://web.archive.org/web/20160613195948/https://tools.ietf.org/html/rfc1345 |url-status=live }}</ref> Consequently, this style may be seen in early [[Internet Engineering Task Force]] documents.

==Notes==
{{notelist}}

== References ==
{{reflist}}

==External links==
* {{cite web|author=Bob Bemer |url=http://www.trailing-edge.com/~bobbemer/BACSLASH.HTM| title=How ASCII got its backslash |archive-url=https://web.archive.org/web/20130719181924/http://www.trailing-edge.com/~bobbemer/BACSLASH.HTM |archive-date=2013-07-19}}

{{navbox punctuation}}

[[Category:Punctuation]]
[[Category:Typographical symbols]]

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