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grep: Character Classes and Bracket Expressions

 
 3.2 Character Classes and Bracket Expressions
 =============================================
 
 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 even be erratic.  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.  ⇒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 nominally
 equivalent regular expression: ‘[:epru]’.  Set the ‘POSIXLY_CORRECT’
 environment variable to disable this feature.
 
    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.
 
 ‘^’
      represents the characters not in the list.  If you want to make the
      ‘^’ character a list item, place it anywhere but first.
 

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