File: grep.info,  Node: Character Classes and Bracket Expressions,  Next: Special Backslash Expressions,  Prev: Fundamental Structure,  Up: Regular Expressions

3.2 Character Classes and Bracket Expressions
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A “bracket expression” is a list of characters enclosed by ‘[’ and ‘]’.
It matches any single character in that list.  If the first character of
the list is the caret ‘^’, then it matches any character *not* in the
list, and it is unspecified whether it matches an encoding error.  For
example, the regular expression ‘[0123456789]’ matches any single digit,
whereas ‘[^()]’ matches any single character that is not an opening or
closing parenthesis, and might or might not match an encoding error.

   Within a bracket expression, a “range expression” consists of two
characters separated by a hyphen.  It matches any single character that
sorts between the two characters, inclusive.  In the default C locale,
the sorting sequence is the native character order; for example, ‘[a-d]’
is equivalent to ‘[abcd]’.  In other locales, the sorting sequence is
not specified, and ‘[a-d]’ might be equivalent to ‘[abcd]’ or to
‘[aBbCcDd]’, or it might fail to match any character, or the set of
characters that it matches might be erratic, or it might be invalid.  To
obtain the traditional interpretation of bracket expressions, you can
use the ‘C’ locale by setting the ‘LC_ALL’ environment variable to the
value ‘C’.

   Finally, certain named classes of characters are predefined within
bracket expressions, as follows.  Their interpretation depends on the
‘LC_CTYPE’ locale; for example, ‘[[:alnum:]]’ means the character class
of numbers and letters in the current locale.

‘[:alnum:]’
     Alphanumeric characters: ‘[:alpha:]’ and ‘[:digit:]’; in the ‘C’
     locale and ASCII character encoding, this is the same as
     ‘[0-9A-Za-z]’.

‘[:alpha:]’
     Alphabetic characters: ‘[:lower:]’ and ‘[:upper:]’; in the ‘C’
     locale and ASCII character encoding, this is the same as
     ‘[A-Za-z]’.

‘[:blank:]’
     Blank characters: space and tab.

‘[:cntrl:]’
     Control characters.  In ASCII, these characters have octal codes
     000 through 037, and 177 (DEL). In other character sets, these are
     the equivalent characters, if any.

‘[:digit:]’
     Digits: ‘0 1 2 3 4 5 6 7 8 9’.

‘[:graph:]’
     Graphical characters: ‘[:alnum:]’ and ‘[:punct:]’.

‘[:lower:]’
     Lower-case letters; in the ‘C’ locale and ASCII character encoding,
     this is ‘a b c d e f g h i j k l m n o p q r s t u v w x y z’.

‘[:print:]’
     Printable characters: ‘[:alnum:]’, ‘[:punct:]’, and space.

‘[:punct:]’
     Punctuation characters; in the ‘C’ locale and ASCII character
     encoding, this is ‘! " # $ % & ' ( ) * + , - . / : ; < = > ? @ [ \
     ] ^ _ ` { | } ~’.

‘[:space:]’
     Space characters: in the ‘C’ locale, this is tab, newline, vertical
     tab, form feed, carriage return, and space.  *Note Usage::, for
     more discussion of matching newlines.

‘[:upper:]’
     Upper-case letters: in the ‘C’ locale and ASCII character encoding,
     this is ‘A B C D E F G H I J K L M N O P Q R S T U V W X Y Z’.

‘[:xdigit:]’
     Hexadecimal digits: ‘0 1 2 3 4 5 6 7 8 9 A B C D E F a b c d e f’.

   Note that the brackets in these class names are part of the symbolic
names, and must be included in addition to the brackets delimiting the
bracket expression.

   If you mistakenly omit the outer brackets, and search for say,
‘[:upper:]’, GNU ‘grep’ prints a diagnostic and exits with status 2, on
the assumption that you did not intend to search for the regular
expression ‘[:epru]’.

   Special characters lose their special meaning inside bracket
expressions.

‘]’
     ends the bracket expression if it's not the first list item.  So,
     if you want to make the ‘]’ character a list item, you must put it
     first.

‘[.’
     represents the open collating symbol.

‘.]’
     represents the close collating symbol.

‘[=’
     represents the open equivalence class.

‘=]’
     represents the close equivalence class.

‘[:’
     represents the open character class symbol, and should be followed
     by a valid character class name.

‘:]’
     represents the close character class symbol.

‘-’
     represents the range if it's not first or last in a list or the
     ending point of a range.  To make the ‘-’ a list item, it is best
     to put it last.

‘^’
     represents the characters not in the list.  If you want to make the
     ‘^’ character a list item, place it anywhere but first.