Support for trigraphs was removed in C++17. Since this can escape the next character, which may itself be a ?, the primary modern use may be for code obfuscation. To support computers that lacked the backslash character, the C trigraph ?/ was added, which is equivalent to a backslash.
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Some software replaces the backslash+newline with a space. In this context it may be called a "continued line" as the current line continues into the next one. Outside quoted strings, the only common use of backslash is to ignore ("escape") a newline immediately after it. 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". Regular expression languages used it the same way, changing subsequent literal characters into metacharacters and vice versa. An actual backslash is produced by a double backslash \\.
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. 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 normally), or normally (if it would otherwise be treated specially).
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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. These operators were used for min and max in early versions of the C programming language supplied with Unix V6. The Committee adopted these changes into the draft American Standard (subsequently called ASCII) at its November 1961 meeting.
In particular, he said, the \ was needed so that the ALGOL boolean operators ∧ ( logical conjunction) and ∨ ( logical disjunction) could be composed using /\ and \/ respectively.
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 June 1960, IBM published an "Extended character set standard" that includes the symbol at 0x19. (This is the code for the slash symbol, entered backwards.) The symbol was called the "diagonal key", and given a (non-standard) Morse code of ▄ ▄▄▄ ▄ ▄ ▄▄▄ .
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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. Teletype ASR-33 keyboard layout with ASCII character set, prior to June 14, 1966, with backslash on shift+LĪs of November 2021, efforts to identify either the origin of this character or its purpose before the 1960s have not been successful.
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