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Wikipedia

Backslash

April 20, 2023

Not to be confused with Backlash, ₩, or ¥.
For technical reasons, ":\" redirects here. For the keyboard symbol, see List of emoticons.

The backslash \ is a typographical mark used mainly in computing and mathematics. It is the mirror image of the common slash /. It is a relatively recent mark, first documented in the 1930s. It is sometimes called a hack, whack, escape (from C/UNIX), reverse slash, slosh, downwhack, backslant, backwhack, bash, reverse slant, and reversed virgule.[1][2]

\
Backslash
In UnicodeU+005C \ REVERSE SOLIDUS (\)
Related
See alsoU+29F5 ⧵ REVERSE SOLIDUS OPERATOR

U+29F9 ⧹ BIG REVERSE SOLIDUS

U+FF3C ＼ FULLWIDTH REVERSE SOLIDUS

History

 
A Teletype Wheatstone Perforator keyboard from the 1930s, with backslash in the end of the third row
 
Teletype ASR-33 keyboard layout with ASCII character set, prior to June 14, 1966, with backslash on shift+L

As of November 2022, 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.[3][4] The symbol was called the "diagonal key",[5] and given a (non-standard) Morse code of   ▄ ▄▄▄ ▄ ▄ ▄▄▄ .[6] (This is the code for the slash symbol, entered backwards.)

In June 1960, IBM published an "Extended character set standard" that includes the symbol at 0x19.[4] In September 1961, Bob Bemer (IBM) proposed to the X3.2 standards committee that [, ] and \ 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 \ was needed so that the ALGOL boolean operators ∧ (logical conjunction) and ∨ (logical disjunction) could be composed using /\ and \/ respectively. The Committee adopted these changes into the draft American Standard (subsequently called ASCII) at its November 1961 meeting.[4]

These operators were used for min and max in early versions of the C programming language supplied with Unix V6[7] and V7.[8]

The Teletype Model 33 (1963) appears to be the first commercially available unit that has the character as a standard keytop for sale in some markets; this model has a full ASCII character set.

Usage

Programming languages

In many programming languages such as C, Perl, PHP, 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 \n produces a newline byte instead of an 'n', and the sequence \" 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 \\.

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

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"[9] as the current line continues into the next one. Some software replaces the backslash+newline with a space.[10]

To support computers that lacked the backslash character, the C trigraph ??/ was added, which is equivalent to a backslash. Since this can escape the next character, which may itself be a ?, the primary modern use may be for code obfuscation. Support for trigraphs in C++ was removed in C++17, though it remains in C.

In Visual Basic (and some other BASIC dialects) the backslash is used as an operator symbol to indicate integer division.[11] This rounds toward zero.

The ALGOL 68 programming language uses the "\" as its Decimal Exponent Symbol. ALGOL 68 has the choice of 4 Decimal Exponent Symbols: e, E, \, or 10. Examples: 6.0221415e23, 6.0221415E23, 6.0221415\23 or 6.02214151023.[12]

In APL \ 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.[13]

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).[14]

Filenames

MS-DOS 2.0, released 1983, copied the idea of a hierarchical file system from Unix and thus used the (forward) slash as the directory separator.[15] Possibly on the insistence of IBM,[16][17] 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 / was the command-line option indicator. Typing "DIR/W" gave the "wide" option to the "DIR" command, so some other method was needed if one actually wanted to run a program called W inside a directory called DIR). Except for COMMAND.COM, all other parts of the operating system accept both characters in a path, but the Microsoft convention remains to use a backslash, and APIs that return paths use backslashes.[18] In some versions, the option character can be changed from / to - via SWITCHAR, which allows COMMAND.COM to preserve / 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 double-quotes.[19] 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 http://example.net/secure\private.aspx.[20][21]

Text markup

The backslash is used in the TeX typesetting system and in RTF files to begin markup tags.

In USFM,[22] the backslash is used to mark format features for editing Bible translations.

In caret notation, ^\ represents the control character 0x1C, file separator.[23] 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 set difference.[24]

The backslash is also sometimes used to denote the right coset space.[25]

Especially when describing computer algorithms, it is common to define backslash so that a\b is equivalent to ⌊a/b⌋.[citation needed] This is integer division that rounds down, not towards zero. In Wolfram Mathematica the backslash is used this way for integer divide.[26]

In MATLAB and GNU Octave the backslash is used for left matrix divide, while the (forward) slash is for right matrix divide.[27]

Confusion with ¥ and other characters

In the Japanese encodings 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 ¥. 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 points 00A5 (¥) and 005C (\) both render as ¥ 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.[28]

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 // 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".[29] Consequently, this style may be seen in early Internet Engineering Task Force documents.

See also

References

  1. ^ Macquarie Dictionary (3rd edition)
  2. ^ Raymond, Eric S. "ASCII". The Jargon File.
  3. ^ Bulletin 125, issue 2: Description and Adjustments of the Teletype Wheatstone Perforator (PDF). Teletype Corporation. May 1938 [August 1937]. p. ii – via navy-radio.com.
  4. ^ a b c Fischer, Eric (20 June 2000). "The Evolution of Character Codes, 1874–1968" (PDF). p. 14–15 (Table 35). Referencing Bemer, R.W.; Buchholz, W. (June 1, 1960). An Extended Character Set Standard (Technical report). Product Development Laboratory, Data Systems Division, International Business Machines Corporation. TR 00.721. Computer Standards Collection, Archives Center, National Museum of American History, Smithsonian Institution, box 1.
  5. ^ Bulletin 188: Teletype automatic perforator set (PDF). Teletype Corporation. August 1945 – via navy-radio.com.
  6. ^ Bulletin 1025, issue 3: Parts for Teletype Wheatstone Perforator (PDF). Teletype Corporation. July 1945 – via navy-radio.com.
  7. ^ "C compiler source". bitsavers.org. 1975.
  8. ^ "C compiler source". mit.edu. 1979.
  9. ^ "The C Preprocessor". gnu.org.
  10. ^ "3.1.1 Splitting long lines". GNU make manual. Retrieved July 28, 2019.
  11. ^ "Arithmetic Operators in Visual Basic". Visual Basic Language Features: Operators and Expressions. MSDN. Retrieved 7 October 2012.
  12. ^ "Revised Report on the Algorithmic Language Algol 68". Acta Informatica. 5 (1–3): 1–236. September 1973. doi:10.1007/BF00265077. S2CID 2490556.
  13. ^ "Namespaces overview". php.net.
  14. ^ O'Sullivan, Stewart, and Goerzen, Real World Haskell, ch. 4: anonymous (lambda) functions, p.99
  15. ^ "Why is the DOS path character "\"?". Blogs.msdn.com. 2005-06-24. Retrieved 2013-06-16.
  16. ^ Necasek, Michal (24 May 2019). . OS/2 Museum. Archived from the original on 24 May 2019. Retrieved 28 May 2019.
  17. ^ Larson, Chris; Immerwahr, Don (1983), MS-DOS v1.25 and v2.0 Source Code, README.txt, Microsoft, retrieved 21 November 2022, 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.
  18. ^ "Path.GetFullPath Method". .NET Framework Class Library. Microsoft. from the original on 21 December 2008. Retrieved 2009-01-02.
  19. ^ "When did Windows start accepting forward slash as a path separator?". Bytes.com. from the original on 6 February 2009. Retrieved 2009-01-02.
  20. ^ Kaplan, Simone (2004). "Microsoft Probes Flaw in ASP.NET". DevSource, sponsored by Microsoft. Ziff Davis Enterprise Holdings Inc. Archived from the original on 2013-01-21. Retrieved 2009-06-14.
  21. ^ Burnett, Mark (2004). "Security Holes That Run Deep". SecurityFocus. Retrieved 2009-06-14.
  22. ^ "USFM – Unified Standard Format Markers". paratext.org. 18 April 2017.
  23. ^ "C0 control characters set, 1987". pd.infn.it.
  24. ^ "Quantities and units – Part 2: Mathematical signs and symbols to be used in the natural sciences and technology". ISO 80000-2:2009. International Organization for Standardization.
  25. ^ "Definition:Coset Space". ProofWiki. Retrieved 1 February 2017.
  26. ^ "Integer Division". mathworld.wolfram.com.
  27. ^ Eaton, John W.; David Bateman; Søren Hauberg (February 2011). "GNU Octave: A high-level interactive language for numerical computations" (PDF). Free Software Foundation. p. 145. Retrieved 7 October 2012.
  28. ^ "When is a backslash not a backslash?". Blogs.msdn.com. Retrieved 2013-06-16.
  29. ^ Simonsen, Keld (1991-12-26). "RFC 1345". Tools.ietf.org. Retrieved 2013-06-16.

External links

  • Bob Bemer. . Archived from the original on 2013-07-19.

April 20, 2023
backslash, confused, with, backlash, technical, reasons, redirects, here, keyboard, symbol, list, emoticons, backslash, typographical, mark, used, mainly, computing, mathematics, mirror, image, common, slash, relatively, recent, mark, first, documented, 1930s,. Not to be confused with Backlash or For technical reasons redirects here For the keyboard symbol see List of emoticons The backslash is a typographical mark used mainly in computing and mathematics It is the mirror image of the common slash It is a relatively recent mark first documented in the 1930s It is sometimes called a hack whack escape from C UNIX reverse slash slosh downwhack backslant backwhack bash reverse slant and reversed virgule 1 2 BackslashIn UnicodeU 005C REVERSE SOLIDUS amp bsol RelatedSee alsoU 29F5 REVERSE SOLIDUS OPERATOR U 29F9 BIG REVERSE SOLIDUS U FF3C FULLWIDTH REVERSE SOLIDUS Contents 1 History 2 Usage 2 1 Programming languages 2 2 Filenames 2 3 Text markup 2 4 Mathematics 3 Confusion with and other characters 4 See also 5 References 6 External linksHistory Edit A Teletype Wheatstone Perforator keyboard from the 1930s with backslash in the end of the third row Teletype ASR 33 keyboard layout with ASCII character set prior to June 14 1966 with backslash on shift L As of November 2022 update 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 3 4 The symbol was called the diagonal key 5 and given a non standard Morse code of 6 This is the code for the slash symbol entered backwards In June 1960 IBM published an Extended character set standard that includes the symbol at 0x19 4 In September 1961 Bob Bemer IBM proposed to the X3 2 standards committee that and 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 was needed so that the ALGOL boolean operators logical conjunction and logical disjunction could be composed using and respectively The Committee adopted these changes into the draft American Standard subsequently called ASCII at its November 1961 meeting 4 These operators were used for min and max in early versions of the C programming language supplied with Unix V6 7 and V7 8 The Teletype Model 33 1963 appears to be the first commercially available unit that has the character as a standard keytop for sale in some markets this model has a full ASCII character set Usage EditProgramming languages Edit In many programming languages such as C Perl PHP 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 n produces a newline byte instead of an n and the sequence 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 Regular expression languages used it the same way changing subsequent literal characters into metacharacters and vice versa For instance b searches for either or b the first bar is escaped and searched for the second is not escaped and acts as an or 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 9 as the current line continues into the next one Some software replaces the backslash newline with a space 10 To support computers that lacked the backslash character the C trigraph was added which is equivalent to a backslash Since this can escape the next character which may itself be a the primary modern use may be for code obfuscation Support for trigraphs in C was removed in C 17 though it remains in C In Visual Basic and some other BASIC dialects the backslash is used as an operator symbol to indicate integer division 11 This rounds toward zero The ALGOL 68 programming language uses the as its Decimal Exponent Symbol ALGOL 68 has the choice of 4 Decimal Exponent Symbols e E or 10 Examples 6 0221415e23 6 0221415E23 6 0221415 23 or 6 02214151023 12 In APL 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 13 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 l 14 Filenames Edit MS DOS 2 0 released 1983 copied the idea of a hierarchical file system from Unix and thus used the forward slash as the directory separator 15 Possibly on the insistence of IBM 16 17 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 was the command line option indicator Typing DIR W gave the wide option to the DIR command so some other method was needed if one actually wanted to run a program called W inside a directory called DIR Except for COMMAND COM all other parts of the operating system accept both characters in a path but the Microsoft convention remains to use a backslash and APIs that return paths use backslashes 18 In some versions the option character can be changed from to via SWITCHAR which allows COMMAND COM to preserve 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 double quotes 19 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 http example net secure private aspx 20 21 Text markup Edit The backslash is used in the TeX typesetting system and in RTF files to begin markup tags In USFM 22 the backslash is used to mark format features for editing Bible translations In caret notation represents the control character 0x1C file separator 23 This is entirely a coincidence and has nothing to do with its use in file paths Mathematics Edit A backslash like symbol is used for the set difference 24 The backslash is also sometimes used to denote the right coset space 25 Especially when describing computer algorithms it is common to define backslash so that a b is equivalent to a b citation needed This is integer division that rounds down not towards zero In Wolfram Mathematica the backslash is used this way for integer divide 26 In MATLAB and GNU Octave the backslash is used for left matrix divide while the forward slash is for right matrix divide 27 Confusion with and other characters EditIn the Japanese encodings 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 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 points 00A5 and 005C both render as 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 28 Several other ISO 646 versions also replace backslash with other characters including Korean O German Swedish O Danish Norwegian c French and N Spanish leading to similar problems though with less lasting impact compared to the yen sign In 1991 RFC 1345 suggested 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 29 Consequently this style may be seen in early Internet Engineering Task Force documents See also EditSlash or solidus References Edit Macquarie Dictionary 3rd edition Raymond Eric S ASCII The Jargon File Bulletin 125 issue 2 Description and Adjustments of the Teletype Wheatstone Perforator PDF Teletype Corporation May 1938 August 1937 p ii via navy radio com a b c Fischer Eric 20 June 2000 The Evolution of Character Codes 1874 1968 PDF p 14 15 Table 35 Referencing Bemer R W Buchholz W June 1 1960 An Extended Character Set Standard Technical report Product Development Laboratory Data Systems Division International Business Machines Corporation TR 00 721 Computer Standards Collection Archives Center National Museum of American History Smithsonian Institution box 1 Bulletin 188 Teletype automatic perforator set PDF Teletype Corporation August 1945 via navy radio com Bulletin 1025 issue 3 Parts for Teletype Wheatstone Perforator PDF Teletype Corporation July 1945 via navy radio com C compiler source bitsavers org 1975 C compiler source mit edu 1979 The C Preprocessor gnu org 3 1 1 Splitting long lines GNU make manual Retrieved July 28 2019 Arithmetic Operators in Visual Basic Visual Basic Language Features Operators and Expressions MSDN Retrieved 7 October 2012 Revised Report on the Algorithmic Language Algol 68 Acta Informatica 5 1 3 1 236 September 1973 doi 10 1007 BF00265077 S2CID 2490556 Namespaces overview php net O Sullivan Stewart and Goerzen Real World Haskell ch 4 anonymous lambda functions p 99 Why is the DOS path character Blogs msdn com 2005 06 24 Retrieved 2013 06 16 Necasek Michal 24 May 2019 Why Does Windows Really Use Backslash as Path Separator OS 2 Museum Archived from the original on 24 May 2019 Retrieved 28 May 2019 Larson Chris Immerwahr Don 1983 MS DOS v1 25 and v2 0 Source Code README txt Microsoft retrieved 21 November 2022 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 Path GetFullPath Method NET Framework Class Library Microsoft Archived from the original on 21 December 2008 Retrieved 2009 01 02 When did Windows start accepting forward slash as a path separator Bytes com Archived from the original on 6 February 2009 Retrieved 2009 01 02 Kaplan Simone 2004 Microsoft Probes Flaw in ASP NET DevSource sponsored by Microsoft Ziff Davis Enterprise Holdings Inc Archived from the original on 2013 01 21 Retrieved 2009 06 14 Burnett Mark 2004 Security Holes That Run Deep SecurityFocus Retrieved 2009 06 14 USFM Unified Standard Format Markers paratext org 18 April 2017 C0 control characters set 1987 pd infn it Quantities and units Part 2 Mathematical signs and symbols to be used in the natural sciences and technology ISO 80000 2 2009 International Organization for Standardization Definition Coset Space ProofWiki Retrieved 1 February 2017 Integer Division mathworld wolfram com Eaton John W David Bateman Soren Hauberg February 2011 GNU Octave A high level interactive language for numerical computations PDF Free Software Foundation p 145 Retrieved 7 October 2012 When is a backslash not a backslash Blogs msdn com Retrieved 2013 06 16 Simonsen Keld 1991 12 26 RFC 1345 Tools ietf org Retrieved 2013 06 16 External links EditBob Bemer How ASCII got its backslash Archived from the original on 2013 07 19 Retrieved from https en wikipedia org w index php title Backslash amp oldid 1141882455, wikipedia, wiki, book, books, library,

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