Bracket Expression and Character Classes

Bracket expressions are a list of characters and/or character classes enclosed in brackets []. Use bracket expressions to match single characters in a list, or a range of characters in a list. If the first character of the list is the carat ^ then it matches characters that are not in the list.

For example:

EXPRESSION	MATCHES
[abc]	a, b, or c
[a-z]	a through z
[^abc]	Any character except a, b, or c
[[:alpha:]]	Any alphabetic character (see below)

The following character classes must be within a bracket expression or it will be treated as a common expression.

CHARACTER CLASS	DESCRIPTION
[:alpha:]	Alphabetic characters
[:digit:]	Digits
[:alnum:]	Alphabetic characters and numeric characters
[:cntrl:]	Control character
[:blank:]	Space and tab
[:space:]	All white space characters
[:graph:]	Non-blank (not spaces, control characters, or the like)
[:print:]	Like [:graph:], but includes the space character
[:punct:]	Punctuation characters
[:lower:]	Lowercase alphabetic character
[:upper:]	Uppercase alphabetic character
[:xdigit:]	Digits allowed in a hexadecimal number (0-9a-fA-F)

For example:

a[[:digit:]]b matches "a0b", "a1b", ..., "a9b".
a[:digit:]b matches "a:b", "adb", …, "atb".
[[:digit:]abc] matches any digit or any of "a", "b", and "c".
[abc[:digit:]] matches any digit or any of "a", "b", and "c".

For a case-insensitive expression, [:lower:] and [:upper:] are equivalent to [:alpha:].