posix-regular-expressions.md

POSIX Regular expressions

http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap09.html

See also:

man 7 regex

POSIX specifies are two types of regular expressions:

• basic (RE)
• extended (ERE)

Basic is deprecated, so don't use it if you can avoid it.

Unlike Perl regular expressions which are super powerful, POSIX regexes really are equivalent in power to mathematical regular expressions https://en.wikipedia.org/wiki/Regular_expression#Formal_definition. This makes them much less powerful, and potentially much more efficient to implement, and easier to learn.

Some POSIX utilities such as grep use BREs by default (backwards compatibility) but can use EREs with an option.

Others like sed only allow BREs, so you still need to learn them.

BRE

http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap09.html#tag_09_03

BREs have special:

• ^ and $ anchors
• [ab] character classes, including negated [^ab]
• . matches all
• * repeats last expression: .*, [ab]*.

Predefined character classes

They enclosed in [::] inside a []. Example:

printf 'a\nb\n' | grep -E '[[:alpha:]]'

Full list:

alnum       digit       punct
alpha       graph       space
blank       lower       upper
cntrl       print       xdigit

Perl like character classes such as \s are not present. They may be provided as GNU extensions like \s in sed.

ERE

http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap09.html#tag_09_04

The main difference is that EREs add more Perl-like special characters.

EREs add:

• (a|b) alternation
• a{1,3} repetition count
• a+ at least one
• a? one or zero.

Convenient character classes like \s are still not present.

Examples:

printf 'a\nb\n'   | grep -E '(a|b)'
printf 'a\nb\n'   | grep -E 'a*'
printf 'a\nb\n'   | grep -E 'a?'
printf 'a\nb\n'   | grep -E 'a+'
printf 'a\nb\n'   | grep -E '^a$'
printf 'a\nb\n'   | grep -E 'a{1,2}'
printf 'aa\nab\n' | grep -E '(a)\1'
printf 'a\nb\n'   | grep -E '.'
printf 'a\nb\n'   | grep -E '[[:alpha:]]'
printf 'a\nA\n'   | grep -E '[[:upper:][:lower:]]'`" = $'a\nA' ]