5.1 Character Class

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. For example, the regular expression `[0123456789]' matches any single digit.

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, using the locale's collating sequence and character set. For example, in the default C locale, `[a-d]' is equivalent to `[abcd]'. Many locales sort characters in dictionary order, and in these locales `[a-d]' is typically not equivalent to `[abcd]'; it might be equivalent to `[aBbCcDd]', for example. 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; the interpretation below is that of the C locale, which is the default if no LC_CTYPE locale is specified.

`[:alnum:]'

Alphanumeric characters: `[:alpha:]' and `[:digit:]'.

`[:alpha:]'

Alphabetic characters: `[:lower:]' and `[:upper:]'.

`[: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: 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: ! " # $ % & ' ( ) * + , - . / : ; < = > ? @ [ \ ] ^ _ ` { | } ~.

`[:space:]'

Space characters: tab, newline, vertical tab, form feed, carriage return, and space.

`[:upper:]'

Upper-case letters: 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.

Most metacharacters lose their special meaning inside lists.

`]'

ends the list 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 followed by a valid character class name.

`:]'

represents the close character class followed by a valid character class name.

`-'

represents the range if it's not first or last in a list or the ending point of a range.

`^'

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