Regular Expressions

Overview

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

• . matches any single character.

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

• [...], 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.

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

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

• * matches the preceding element zero or more times.

• + matches the preceding element one or more times.

• ? matches the preceding element zero or one times.

• {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.

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

Character Classes

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

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

Bibliography

• “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