notebook/notes/posix/regexp.md

title	TARGET DECK	FILE TAGS	tags
Regular Expressions	Obsidian::STEM	regexp	regexp

Overview

The following ERE (Extended Regular Expression) operators were defined to achieve consistency between programs like grep, sed, and awk.

%%ANKI Basic What is the POSIX ERE standard? Back: The Extended Regular Expression standard. A standard based off of regexps accepted by egrep. Reference: “POSIX Basic Regular Expressions,” accessed February 4, 2024, https://en.wikibooks.org/wiki/Regular_Expressions/POSIX_Basic_Regular_Expressions.

END%%

• . matches any single character.
  • There exist application-specific exclusions. For instance, newlines and the NUL character are often ignored.

%%ANKI Cloze The {.} operator matches {any single character}. Reference: “POSIX Basic Regular Expressions,” accessed February 4, 2024, https://en.wikibooks.org/wiki/Regular_Expressions/POSIX_Basic_Regular_Expressions.

END%%

%%ANKI Basic What two common exclusions are made with .? Back: Newlines and the NUL character. Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. https://www.gnu.org/software/gawk/manual/gawk.pdf

END%%

• [...], the bracket expression, matches any enclosed character.
  • An optional - can be included to denote a range.
  • - is treated literally if its the first or last specified character.
  • ] is treated literally if its the first specified character.
  • ^ complements the set if its the first specified character.

%%ANKI Basic What name is given to the [...] operator? Back: The bracket expression. Reference: “POSIX Basic Regular Expressions,” accessed February 4, 2024, https://en.wikibooks.org/wiki/Regular_Expressions/POSIX_Basic_Regular_Expressions.

END%%

%%ANKI Basic What three characters are interpreted specially in a bracket expression? Back: ^, -, and ] Reference: “POSIX Basic Regular Expressions,” accessed February 4, 2024, https://en.wikibooks.org/wiki/Regular_Expressions/POSIX_Basic_Regular_Expressions.

END%%

%%ANKI Basic When is - interpreted literally in a bracket expression? Back: When it is the first or last specified character. Reference: “POSIX Basic Regular Expressions,” accessed February 4, 2024, https://en.wikibooks.org/wiki/Regular_Expressions/POSIX_Basic_Regular_Expressions.

END%%

%%ANKI Basic When is ^ interpreted literally in a bracket expression? Back: When it is not the first specified character. Reference: “POSIX Basic Regular Expressions,” accessed February 4, 2024, https://en.wikibooks.org/wiki/Regular_Expressions/POSIX_Basic_Regular_Expressions.

END%%

%%ANKI Basic When is ] interpreted literally in a bracket expression? Back: When it is the first specified character. Reference: “POSIX Basic Regular Expressions,” accessed February 4, 2024, https://en.wikibooks.org/wiki/Regular_Expressions/POSIX_Basic_Regular_Expressions.

END%%

• ^ is the leading anchor. It matches the starting position of a string.
• $ is the trailing anchor. It matches the ending position of a string.

%%ANKI Cloze The {^} operator matches {the starting position of a string}. Reference: “POSIX Basic Regular Expressions,” accessed February 4, 2024, https://en.wikibooks.org/wiki/Regular_Expressions/POSIX_Basic_Regular_Expressions.

END%%

%%ANKI Cloze The {$} operator matches {the ending position of a string}. Reference: “POSIX Basic Regular Expressions,” accessed February 4, 2024, https://en.wikibooks.org/wiki/Regular_Expressions/POSIX_Basic_Regular_Expressions.

END%%

%%ANKI Basic ^ and $ belong to what operator category? Back: Anchors Reference: “POSIX Basic Regular Expressions,” accessed February 4, 2024, https://en.wikibooks.org/wiki/Regular_Expressions/POSIX_Basic_Regular_Expressions.

END%%

• * matches the preceding element zero or more times.
• + matches the preceding element one or more times.
• ? matches the preceding element zero or one times.

%%ANKI Basic What does the * operator do? Back: Matches the preceding element zero or more times. Reference: “POSIX Basic Regular Expressions,” accessed February 4, 2024, https://en.wikibooks.org/wiki/Regular_Expressions/POSIX_Basic_Regular_Expressions.

END%%

%%ANKI Basic What does the + operator do? Back: Matches the preceding element one or more times. Reference: “POSIX Basic Regular Expressions,” accessed February 4, 2024, https://en.wikibooks.org/wiki/Regular_Expressions/POSIX_Basic_Regular_Expressions.

END%%

%%ANKI Basic What does the ? operator do? Back: Matches the preceding element zero or one times. Reference: “POSIX Basic Regular Expressions,” accessed February 4, 2024, https://en.wikibooks.org/wiki/Regular_Expressions/POSIX_Basic_Regular_Expressions.

END%%

• {n}, an interval expression, matches the preceding element n times.
  • {n,} matches the preceding element at least n times.
  • {n,m} matches the preceding element between n and m times.
  • Interval expressions cannot contain repetition counts > 255. Results are otherwise undefined.

%%ANKI Basic What name is given to the e.g. {n,m} operator? Back: The interval expression. Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. https://www.gnu.org/software/gawk/manual/gawk.pdf

END%%

%%ANKI Basic What does the {n} operator do? Back: Matches the preceding element n times. Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. https://www.gnu.org/software/gawk/manual/gawk.pdf

END%%

%%ANKI Basic What does the {n,} operator do? Back: Matches the preceding element at least n times. Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. https://www.gnu.org/software/gawk/manual/gawk.pdf

END%%

%%ANKI Basic What does the {n,m} operator do? Back: Matches the preceding element between n and m times. Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. https://www.gnu.org/software/gawk/manual/gawk.pdf

END%%

%%ANKI Basic What interval expression repetition counts lead to undefined behavior? Back: Counts greater than 255. Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. https://www.gnu.org/software/gawk/manual/gawk.pdf

END%%

• | is the alternation operator. It allows specifying match alternatives.

%%ANKI Basic What name is given to the e.g. | operator? Back: The alternation operator. Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. https://www.gnu.org/software/gawk/manual/gawk.pdf

END%%

%%ANKI Basic What does the | operator do? Back: Matches different regexp alternatives. Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. https://www.gnu.org/software/gawk/manual/gawk.pdf

END%%

%%ANKI Basic Which regexp operator has the least precedence? Back: | Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. https://www.gnu.org/software/gawk/manual/gawk.pdf

END%%

Character Classes

Notation for describing a class of characters specific to a given locale/character set.

%%ANKI Basic What inconsistency do character classes introduce? Back: Matching characters are dependent on locale/character set. Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. https://www.gnu.org/software/gawk/manual/gawk.pdf

END%%

%%ANKI Basic How are character classes denoted? Back: [:class:] Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. https://www.gnu.org/software/gawk/manual/gawk.pdf

END%%

Class	Similar To	Meaning
[:alnum:]	[A-Za-z0-9]	Alphanumeric characters
[:alpha:]	[A-Za-z]	Alphabetic characters
[:blank:]	[ \t]	' ' and TAB characters
[:cntrl:]		Control characters
[:digit:]	[0-9]	Numeric characters
[:graph:]	[^ [:cntrl:]]	Printable and visible characters
[:lower:]	[a-z]	Lowercase alphabetic characters
[:print:]	[ [:graph:]]	Printable characters
[:punct:]		All graphic characters except letters and digits
[:space:]	[ \t\n\r\f\v]	Whitespace characters
[:upper:]	[A-Z]	Uppercase alphabetic characters
[:xdigit:]	[0-9A-Fa-f]	Hexadecimal digits

%%ANKI Basic Generally speaking, what is a printable character? Back: Characters that can be displayed on screen or printed on paper. Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. https://www.gnu.org/software/gawk/manual/gawk.pdf

END%%

%%ANKI Basic Is 'a' (i.e. the letter a) printable and/or visible? Back: It is printable and visible. Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. https://www.gnu.org/software/gawk/manual/gawk.pdf

END%%

%%ANKI Basic Is ' ' (i.e. the space character) printable and/or visible? Back: It is printable but not visible. Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. https://www.gnu.org/software/gawk/manual/gawk.pdf

END%%

%%ANKI Basic Is '\t' (i.e. the tab character) printable and/or visible? Back: It is neither printable nor visible. Reference: Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. https://www.gnu.org/software/gawk/manual/gawk.pdf

END%%

References

• “POSIX Basic Regular Expressions,” accessed February 4, 2024, https://en.wikibooks.org/wiki/Regular_Expressions/POSIX_Basic_Regular_Expressions.
• Robbins, Arnold D. “GAWK: Effective AWK Programming,” October 2023. https://www.gnu.org/software/gawk/manual/gawk.pdf