148 lines
5.7 KiB
Markdown
148 lines
5.7 KiB
Markdown
---
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title: Escape Sequences
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TARGET DECK: Obsidian::STEM
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FILE TAGS: c17
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tags:
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- c17
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---
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## Overview
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C has a standard for processing different escape sequences. Many languages built with C in mind parse these escape sequences in a similar way.
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* `\ooo`: Consists of one to three octal digits.
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* [[bash/index|Bash]] supports this sequence as `$'\ooo'`.
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* [[gawk/index|gawk]] supports this sequence directly.
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* [[lua/index|Lua]] does not support this kind of escape sequence. Instead, it has a *decimal* escape sequence `\ddd`.
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%%ANKI
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Basic
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How are C escape sequences for octal digits denoted?
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Back: As `\ooo`.
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Reference: Brian W. Kernighan and Dennis M. Ritchie, *The C Programming Language*, 2nd ed (Englewood Cliffs, N.J: Prentice Hall, 1988).
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<!--ID: 1706975891805-->
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END%%
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%%ANKI
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Basic
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In C, `\ooo` allows specifying how many octal digits?
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Back: One to three.
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Reference: Brian W. Kernighan and Dennis M. Ritchie, *The C Programming Language*, 2nd ed (Englewood Cliffs, N.J: Prentice Hall, 1988).
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<!--ID: 1706975891810-->
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END%%
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%%ANKI
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Basic
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What alternative does Lua provide to C's `\ooo` sequence?
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Back: `\ddd`, a *decimal* escape sequence.
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Reference: Roberto Ierusalimschy, _Programming in Lua_, Fourth edition (Rio de Janeiro: Lua.org, 2016).
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Tags: lua
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<!--ID: 1706975891813-->
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END%%
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%%ANKI
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Basic
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How are C escape sequences exposed in bash?
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Back: Using ANSI-C quoting, i.e. `$$'string'`.
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Reference: Mendel Cooper, “Advanced Bash-Scripting Guide,” n.d., 916.
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Tags: bash
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<!--ID: 1706975891817-->
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END%%
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* `\xhh`: Consists of one or more [[radices#Hexadecimal|hexadecimal]] digits. The `x` prefix is required to distinguish from octal escape sequences.
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* [[bash/index|Bash]] supports this sequence as `$'\xhh'`. One or two digits is supported.
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* [[gawk/index|gawk]] limits processing to two digits.
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* Robbins states that using more than two hexadecimal digits can produce undefined results.
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* [[Lua/index|Lua]] requires *exactly* two digits in its hex escape sequence.
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%%ANKI
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Basic
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How are C escape sequences for hexadecimal digits denoted?
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Back: As `\xhh`.
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Reference: Brian W. Kernighan and Dennis M. Ritchie, *The C Programming Language*, 2nd ed (Englewood Cliffs, N.J: Prentice Hall, 1988).
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<!--ID: 1706975891820-->
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END%%
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%%ANKI
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Basic
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In C, `\x` allows specifying how many hexadecimal digits?
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Back: One or more.
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Reference: Brian W. Kernighan and Dennis M. Ritchie, *The C Programming Language*, 2nd ed (Englewood Cliffs, N.J: Prentice Hall, 1988).
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<!--ID: 1706975891824-->
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END%%
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%%ANKI
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Basic
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What footgun does C's `\x` sequence expose?
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Back: Using more than two hexadecimal digits can produce undefined results.
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Reference: Arnold D. Robbins, “GAWK: Effective AWK Programming,” October 2023, [https://www.gnu.org/software/gawk/manual/gawk.pdf](https://www.gnu.org/software/gawk/manual/gawk.pdf).
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<!--ID: 1706975891828-->
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END%%
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* `\uhhhh`: Introduced in C11 to represent Unicode code points. *Must* have exactly four hexadecimal characters specified with `0` leading padding if necessary.
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* [[bash/index|Bash]] supports this sequence as `$'uhhhh'`. One to four hex digits is supported.
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* [[gawk/index|gawk]] 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.
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* [[lua/index|Lua]] 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.
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%%ANKI
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Basic
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What two ways are C escape sequences for unicode denoted?
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Back: As `\uhhhh` or `\Uhhhhhhhh`.
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Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
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Tags: unicode
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<!--ID: 1706975891832-->
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END%%
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%%ANKI
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Basic
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In C, `\u` allows specifying how many hexadecimal digits?
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Back: Exactly four.
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Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
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Tags: unicode
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<!--ID: 1706975891835-->
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END%%
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%%ANKI
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Basic
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In what standard were C's `\u` and `\U` escape sequences introduced?
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Back: C11.
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Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
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Tags: unicode
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<!--ID: 1706975891839-->
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END%%
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%%ANKI
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Cloze
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`\u` in C designates a character in {Unicode}. In `gawk` it designates a character in {the current locale's character set}.
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Reference: Arnold D. Robbins, “GAWK: Effective AWK Programming,” October 2023, [https://www.gnu.org/software/gawk/manual/gawk.pdf](https://www.gnu.org/software/gawk/manual/gawk.pdf).
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Tags: unicode gawk
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<!--ID: 1706976541399-->
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END%%
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* `\Uhhhhhhhh`: Introduced in C11 to represent larger unicode code points. *Must* have exactly eight hexadecimal characters specified with `0` leading padding if necessary.
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%%ANKI
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Basic
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In C, `\U` allows specifying how many hexadecimal digits?
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Back: Exactly eight.
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Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
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Tags: unicode
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<!--ID: 1706975891843-->
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END%%
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%%ANKI
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Basic
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Why does C have both `\u` and `\U`?
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Back: `\U` accommodates for larger code point values.
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Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
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Tags: unicode
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<!--ID: 1706976705750-->
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END%%
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## References
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* Arnold D. Robbins, “GAWK: Effective AWK Programming,” October 2023, [https://www.gnu.org/software/gawk/manual/gawk.pdf](https://www.gnu.org/software/gawk/manual/gawk.pdf).
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* Brian W. Kernighan and Dennis M. Ritchie, *The C Programming Language*, 2nd ed (Englewood Cliffs, N.J: Prentice Hall, 1988).
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* Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
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* Mendel Cooper, “Advanced Bash-Scripting Guide,” n.d., 916.
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* Roberto Ierusalimschy, _Programming in Lua_, Fourth edition (Rio de Janeiro: Lua.org, 2016). |