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sed: Character Classes and Bracket Expressions

 
 5.5 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.  For example, the following command replaces the words 'gray' or
 'grey' with 'blue':
 
      sed  's/gr[ae]y/blue/'
 
    Bracket expressions can be used in both ⇒basic BRE syntax. and
 ⇒extended ERE syntax. regular expressions (that is, with or
 without the '-E'/'-r' options).
 
    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]'.
 
    Finally, certain named classes of characters are predefined within
 bracket expressions, as follows.
 
    These named classes must be used _inside_ brackets themselves.
 Correct usage:
      $ echo 1 | sed 's/[[:digit:]]/X/'
      X
 
    Incorrect usage is rejected by newer 'sed' versions.  Older versions
 accepted it but treated it as a single bracket expression (which is
 equivalent to '[dgit:]', that is, only the characters D/G/I/T/:):
      # current GNU sed versions - incorrect usage rejected
      $ echo 1 | sed 's/[:digit:]/X/'
      sed: character class syntax is [[:space:]], not [:space:]
 
      # older GNU sed versions
      $ echo 1 | sed 's/[:digit:]/X/'
      1
 
 '[: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.
 
 '[: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.
 
    Most meta-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 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.
 
    TODO: incorporate this paragraph (copied verbatim from BRE section).
 
    The characters '$', '*', '.', '[', and '\' are normally not special
 within LIST.  For example, '[\*]' matches either '\' or '*', because the
 '\' is not special here.  However, strings like '[.ch.]', '[=a=]', and
 '[:space:]' are special within LIST and represent collating symbols,
 equivalence classes, and character classes, respectively, and '[' is
 therefore special within LIST when it is followed by '.', '=', or ':'.
 Also, when not in 'POSIXLY_CORRECT' mode, special escapes like '\n' and
 '\t' are recognized within LIST.  ⇒Escapes.
 
 '[.'
      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.
 

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