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title TARGET DECK FILE TAGS tags
Escape Sequences Obsidian::STEM c
c

Overview

C has a standard for processing different escape sequences. Many languages built with C in mind parse these escape sequences in a similar way.

  • \ooo: Consists of one to three octal digits.
    • bash/index supports this sequence as $'\ooo'.
    • gawk/index supports this sequence directly.
    • lua/index does not support this kind of escape sequence. Instead, it has a decimal escape sequence \ddd.

%%ANKI Basic How are C escape sequences for octal digits denoted? Back: As \ooo. Reference: Brian W. Kernighan and Dennis M. Ritchie, The C Programming Language, 2nd ed (Englewood Cliffs, N.J: Prentice Hall, 1988).

END%%

%%ANKI Basic In C, \ooo allows specifying how many octal digits? Back: One to three. Reference: Brian W. Kernighan and Dennis M. Ritchie, The C Programming Language, 2nd ed (Englewood Cliffs, N.J: Prentice Hall, 1988).

END%%

%%ANKI Basic What alternative does Lua provide to C's \ooo sequence? Back: \ddd, a decimal escape sequence. Reference: Roberto Ierusalimschy, Programming in Lua, Fourth edition (Rio de Janeiro: Lua.org, 2016). Tags: lua

END%%

%%ANKI Basic How are C escape sequences exposed in bash? Back: Using ANSI-C quoting, i.e. $$'string'. Reference: Mendel Cooper, “Advanced Bash-Scripting Guide,” n.d., 916. Tags: bash

END%%

  • \xhh: Consists of one or more hexadecimal digits. The x prefix is required to distinguish from octal escape sequences.
    • bash/index supports this sequence as $'\xhh'. One or two digits is supported.
    • gawk/index limits processing to two digits.
      • Robbins states that using more than two hexadecimal digits can produce undefined results.
    • Lua/index requires exactly two digits in its hex escape sequence.

%%ANKI Basic How are C escape sequences for hexadecimal digits denoted? Back: As \xhh. Reference: Brian W. Kernighan and Dennis M. Ritchie, The C Programming Language, 2nd ed (Englewood Cliffs, N.J: Prentice Hall, 1988).

END%%

%%ANKI Basic In C, \x allows specifying how many hexadecimal digits? Back: One or more. Reference: Brian W. Kernighan and Dennis M. Ritchie, The C Programming Language, 2nd ed (Englewood Cliffs, N.J: Prentice Hall, 1988).

END%%

%%ANKI Basic What footgun does C's \x sequence expose? Back: Using more than two hexadecimal digits can produce undefined results. Reference: Arnold D. Robbins, “GAWK: Effective AWK Programming,” October 2023, https://www.gnu.org/software/gawk/manual/gawk.pdf.

END%%

  • \uhhhh: Introduced in C11 to represent Unicode code points. Must have exactly four hexadecimal characters specified with 0 leading padding if necessary.
    • bash/index supports this sequence as $'uhhhh'. One to four hex digits is supported.
    • gawk/index consolidates C's \u and \U sequence marker into just \u, capable of handling one to eight digits. Furthermore, gawk uses \u to designate the current locale's character set, not Unicode directly. Often times this is some Unicode-based locale though.
    • lua/index consolidates C's \u and \U sequence markers into \u{h...h}, capable of handling one or more hexadecimal digits. The curly braces are required.

%%ANKI Basic What two ways are C escape sequences for unicode denoted? Back: As \uhhhh or \Uhhhhhhhh. Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020). Tags: unicode

END%%

%%ANKI Basic In C, \u allows specifying how many hexadecimal digits? Back: Exactly four. Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020). Tags: unicode

END%%

%%ANKI Basic In what standard were C's \u and \U escape sequences introduced? Back: C11. Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020). Tags: unicode

END%%

%%ANKI Cloze \u in C designates a character in {Unicode}. In gawk it designates a character in {the current locale's character set}. Reference: Arnold D. Robbins, “GAWK: Effective AWK Programming,” October 2023, https://www.gnu.org/software/gawk/manual/gawk.pdf. Tags: unicode gawk

END%%

  • \Uhhhhhhhh: Introduced in C11 to represent larger unicode code points. Must have exactly eight hexadecimal characters specified with 0 leading padding if necessary.

%%ANKI Basic In C, \U allows specifying how many hexadecimal digits? Back: Exactly eight. Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020). Tags: unicode

END%%

%%ANKI Basic Why does C have both \u and \U? Back: \U accommodates for larger code point values. Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020). Tags: unicode

END%%

References

  • Arnold D. Robbins, “GAWK: Effective AWK Programming,” October 2023, https://www.gnu.org/software/gawk/manual/gawk.pdf.
  • Brian W. Kernighan and Dennis M. Ritchie, The C Programming Language, 2nd ed (Englewood Cliffs, N.J: Prentice Hall, 1988).
  • Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020).
  • Mendel Cooper, “Advanced Bash-Scripting Guide,” n.d., 916.
  • Roberto Ierusalimschy, Programming in Lua, Fourth edition (Rio de Janeiro: Lua.org, 2016).