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---
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title: Regular Expressions
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TARGET DECK: Obsidian::STEM
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FILE TAGS: regexp
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tags:
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- regexp
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---
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## Overview
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The following ERE (**E**xtended **R**egular **E**xpression) operators were defined to achieve consistency between programs like `grep`, `sed`, and `awk`.
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%%ANKI
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Basic
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What is the POSIX ERE standard?
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Back: The **E**xtended **R**egular **E**xpression standard. A standard based off of regexps accepted by `egrep`.
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Reference: “POSIX Basic Regular Expressions,” accessed February 4, 2024, [https://en.wikibooks.org/wiki/Regular_Expressions/POSIX_Basic_Regular_Expressions](https://en.wikibooks.org/wiki/Regular_Expressions/POSIX_Basic_Regular_Expressions).
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<!--ID: 1707050923589-->
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END%%
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* `.` matches any single character.
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* There exist application-specific exclusions. For instance, newlines and the `NUL` character are often ignored.
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%%ANKI
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Cloze
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The {`.`} operator matches {any single character}.
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Reference: “POSIX Basic Regular Expressions,” accessed February 4, 2024, [https://en.wikibooks.org/wiki/Regular_Expressions/POSIX_Basic_Regular_Expressions](https://en.wikibooks.org/wiki/Regular_Expressions/POSIX_Basic_Regular_Expressions).
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<!--ID: 1707050923593-->
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END%%
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%%ANKI
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Basic
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What two common exclusions are made with `.`?
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Back: Newlines and the `NUL` character.
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Reference: Robbins, Arnold D. “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: 1707050923596-->
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END%%
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* `[...]`, the **bracket expression**, matches any enclosed character.
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* An optional `-` can be included to denote a range.
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* `-` is treated literally if its the first or last specified character.
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* `]` is treated literally if its the first specified character.
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* `^` complements the set if its the first specified character.
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%%ANKI
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Basic
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What name is given to the `[...]` operator?
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Back: The bracket expression.
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Reference: “POSIX Basic Regular Expressions,” accessed February 4, 2024, [https://en.wikibooks.org/wiki/Regular_Expressions/POSIX_Basic_Regular_Expressions](https://en.wikibooks.org/wiki/Regular_Expressions/POSIX_Basic_Regular_Expressions).
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<!--ID: 1707050923600-->
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END%%
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%%ANKI
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Basic
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What three characters are interpreted specially in a bracket expression?
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Back: `^`, `-`, and `]`
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Reference: “POSIX Basic Regular Expressions,” accessed February 4, 2024, [https://en.wikibooks.org/wiki/Regular_Expressions/POSIX_Basic_Regular_Expressions](https://en.wikibooks.org/wiki/Regular_Expressions/POSIX_Basic_Regular_Expressions).
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<!--ID: 1707050923605-->
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END%%
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%%ANKI
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Basic
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When is `-` interpreted literally in a bracket expression?
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Back: When it is the first or last specified character.
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Reference: “POSIX Basic Regular Expressions,” accessed February 4, 2024, [https://en.wikibooks.org/wiki/Regular_Expressions/POSIX_Basic_Regular_Expressions](https://en.wikibooks.org/wiki/Regular_Expressions/POSIX_Basic_Regular_Expressions).
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<!--ID: 1707050923610-->
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END%%
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%%ANKI
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Basic
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When is `^` interpreted literally in a bracket expression?
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Back: When it is not the first specified character.
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Reference: “POSIX Basic Regular Expressions,” accessed February 4, 2024, [https://en.wikibooks.org/wiki/Regular_Expressions/POSIX_Basic_Regular_Expressions](https://en.wikibooks.org/wiki/Regular_Expressions/POSIX_Basic_Regular_Expressions).
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<!--ID: 1707050923615-->
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END%%
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%%ANKI
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Basic
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When is `]` interpreted literally in a bracket expression?
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Back: When it is the first specified character.
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Reference: “POSIX Basic Regular Expressions,” accessed February 4, 2024, [https://en.wikibooks.org/wiki/Regular_Expressions/POSIX_Basic_Regular_Expressions](https://en.wikibooks.org/wiki/Regular_Expressions/POSIX_Basic_Regular_Expressions).
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<!--ID: 1707050923621-->
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END%%
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* `^` is the leading anchor. It matches the starting position of a string.
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* `$` is the trailing anchor. It matches the ending position of a string.
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%%ANKI
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Cloze
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The {`^`} operator matches {the starting position of a string}.
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Reference: “POSIX Basic Regular Expressions,” accessed February 4, 2024, [https://en.wikibooks.org/wiki/Regular_Expressions/POSIX_Basic_Regular_Expressions](https://en.wikibooks.org/wiki/Regular_Expressions/POSIX_Basic_Regular_Expressions).
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<!--ID: 1707050923629-->
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END%%
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%%ANKI
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Cloze
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The {`$`} operator matches {the ending position of a string}.
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Reference: “POSIX Basic Regular Expressions,” accessed February 4, 2024, [https://en.wikibooks.org/wiki/Regular_Expressions/POSIX_Basic_Regular_Expressions](https://en.wikibooks.org/wiki/Regular_Expressions/POSIX_Basic_Regular_Expressions).
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<!--ID: 1707050923635-->
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END%%
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%%ANKI
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Basic
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`^` and `$` belong to what operator category?
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Back: Anchors
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Reference: “POSIX Basic Regular Expressions,” accessed February 4, 2024, [https://en.wikibooks.org/wiki/Regular_Expressions/POSIX_Basic_Regular_Expressions](https://en.wikibooks.org/wiki/Regular_Expressions/POSIX_Basic_Regular_Expressions).
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<!--ID: 1707050923643-->
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END%%
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* `*` matches the preceding element zero or more times.
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* `+` matches the preceding element one or more times.
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* `?` matches the preceding element zero or one times.
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%%ANKI
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Basic
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What does the `*` operator do?
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Back: Matches the preceding element zero or more times.
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Reference: “POSIX Basic Regular Expressions,” accessed February 4, 2024, [https://en.wikibooks.org/wiki/Regular_Expressions/POSIX_Basic_Regular_Expressions](https://en.wikibooks.org/wiki/Regular_Expressions/POSIX_Basic_Regular_Expressions).
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<!--ID: 1707050923650-->
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END%%
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%%ANKI
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Basic
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What does the `+` operator do?
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Back: Matches the preceding element one or more times.
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Reference: “POSIX Basic Regular Expressions,” accessed February 4, 2024, [https://en.wikibooks.org/wiki/Regular_Expressions/POSIX_Basic_Regular_Expressions](https://en.wikibooks.org/wiki/Regular_Expressions/POSIX_Basic_Regular_Expressions).
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<!--ID: 1707050923656-->
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END%%
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%%ANKI
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Basic
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What does the `?` operator do?
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Back: Matches the preceding element zero or one times.
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Reference: “POSIX Basic Regular Expressions,” accessed February 4, 2024, [https://en.wikibooks.org/wiki/Regular_Expressions/POSIX_Basic_Regular_Expressions](https://en.wikibooks.org/wiki/Regular_Expressions/POSIX_Basic_Regular_Expressions).
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<!--ID: 1707050923662-->
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END%%
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* `{n}`, an **interval expression**, matches the preceding element `n` times.
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* `{n,}` matches the preceding element at least `n` times.
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* `{n,m}` matches the preceding element between `n` and `m` times.
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* Interval expressions cannot contain repetition counts `> 255`. Results are otherwise undefined.
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%%ANKI
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Basic
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What name is given to the e.g. `{n,m}` operator?
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Back: The interval expression.
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Reference: Robbins, Arnold D. “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: 1707050923669-->
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END%%
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%%ANKI
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Basic
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What does the `{n}` operator do?
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Back: Matches the preceding element `n` times.
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Reference: Robbins, Arnold D. “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: 1707050923676-->
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END%%
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%%ANKI
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Basic
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What does the `{n,}` operator do?
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Back: Matches the preceding element at least `n` times.
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Reference: Robbins, Arnold D. “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: 1707050923683-->
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END%%
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%%ANKI
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Basic
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What does the `{n,m}` operator do?
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Back: Matches the preceding element between `n` and `m` times.
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Reference: Robbins, Arnold D. “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: 1707050923689-->
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END%%
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%%ANKI
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Basic
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What interval expression repetition counts lead to undefined behavior?
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Back: Counts greater than `255`.
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Reference: Robbins, Arnold D. “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: 1707050923695-->
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END%%
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* `|` is the **alternation operator**. It allows specifying match alternatives.
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%%ANKI
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Basic
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What name is given to the e.g. `|` operator?
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Back: The alternation operator.
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Reference: Robbins, Arnold D. “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: 1707050923701-->
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END%%
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%%ANKI
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Basic
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What does the `|` operator do?
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Back: Matches different regexp alternatives.
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Reference: Robbins, Arnold D. “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: 1707050923708-->
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END%%
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%%ANKI
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Basic
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Which regexp operator has the least precedence?
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Back: `|`
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Reference: Robbins, Arnold D. “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: 1707050923713-->
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END%%
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## Character Classes
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Notation for describing a class of characters specific to a given locale/character set.
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%%ANKI
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Basic
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What inconsistency do character classes introduce?
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Back: Matching characters are dependent on locale/character set.
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Reference: Robbins, Arnold D. “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: 1707050923719-->
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END%%
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%%ANKI
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Basic
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How are character classes denoted?
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Back: `[:class:]`
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Reference: Robbins, Arnold D. “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: 1707050923724-->
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END%%
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Class | Similar To | Meaning
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------------ | --------------- | ------------------------------------------------
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`[:alnum:]` | `[A-Za-z0-9]` | Alphanumeric characters
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`[:alpha:]` | `[A-Za-z]` | Alphabetic characters
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`[:blank:]` | `[ \t]` | `' '` and `TAB` characters
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`[:cntrl:]` | | Control characters
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`[:digit:]` | `[0-9]` | Numeric characters
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`[:graph:]` | `[^ [:cntrl:]]` | Printable and visible characters
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`[:lower:]` | `[a-z]` | Lowercase alphabetic characters
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`[:print:]` | `[ [:graph:]]` | Printable characters
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`[:punct:]` | | All graphic characters except letters and digits
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`[:space:]` | `[ \t\n\r\f\v]` | Whitespace characters
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`[:upper:]` | `[A-Z]` | Uppercase alphabetic characters
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`[:xdigit:]` | `[0-9A-Fa-f]` | Hexadecimal digits
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%%ANKI
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Basic
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Generally speaking, what is a printable character?
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Back: Characters that can be displayed on screen or printed on paper.
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Reference: Robbins, Arnold D. “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: 1707050923728-->
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END%%
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%%ANKI
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Basic
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Is `'a'` (i.e. the letter *a*) printable and/or visible?
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Back: It is printable and visible.
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Reference: Robbins, Arnold D. “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: 1707050923732-->
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END%%
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%%ANKI
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Basic
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Is `' '` (i.e. the space character) printable and/or visible?
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Back: It is printable but not visible.
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Reference: Robbins, Arnold D. “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: 1707050923736-->
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END%%
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%%ANKI
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Basic
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Is `'\t'` (i.e. the tab character) printable and/or visible?
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Back: It is neither printable nor visible.
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Reference: Robbins, Arnold D. “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: 1707050923740-->
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END%%
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## References
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* “POSIX Basic Regular Expressions,” accessed February 4, 2024, [https://en.wikibooks.org/wiki/Regular_Expressions/POSIX_Basic_Regular_Expressions](https://en.wikibooks.org/wiki/Regular_Expressions/POSIX_Basic_Regular_Expressions).
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* Robbins, Arnold D. “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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