83 lines
2.9 KiB
Markdown
83 lines
2.9 KiB
Markdown
---
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title: Regular Expressions
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TARGET DECK: Obsidian::STEM
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FILE TAGS: linux::cli gawk regexp
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tags:
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- gawk
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---
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## Overview
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Most `awk` patterns are regular expressions delimited with `/`. We can use `~` and `!~` to perform more complicated regexp filtering:
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```bash
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# Matches any line with `li` somewhere.
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$ awk '/li/' data
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$ awk `$0 ~ /li/` data
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# Matches any line with `li` somewhere in the first field.
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$ awk '$1 ~ /li/' data
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```
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`awk`'s implementation of regexps are a superset of [[posix/regexp|POSIX EREs]].
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%%ANKI
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Basic
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What is the result of the following?
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```bash
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$ echo aaaabcd | awk '{ sub(/a+/, "<A>"); print }'
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```
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Back: `<A>bcd`
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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: 1707231831794-->
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END%%
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%%ANKI
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Cloze
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In `awk`, `/.../` is to a {regexp} constant whereas `"..."` is to a {string} constant.
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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: 1707232573706-->
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END%%
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%%ANKI
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Basic
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How are string constants processed differently from regexp constants?
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Back: The string constant is scanned twice.
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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: 1707232573708-->
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END%%
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%%ANKI
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Basic
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What term describes a regexp that isn't a regexp constant?
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Back: A dynamic regexp.
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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: 1707232573710-->
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END%%
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%%ANKI
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Basic
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How is `*` escaped in a regexp constant?
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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: 1707232573712-->
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END%%
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%%ANKI
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Basic
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How is `*` escaped in a string constant (dynamic regexp)?
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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: 1707232573713-->
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END%%
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%%ANKI
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Basic
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Why is it recommended to avoid using `^`/`$` in `RS`?
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Back: These anchors match the beginning/end of a string, not of a line.
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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: 1707310981375-->
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END%%
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## References
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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) |