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 GNU Extensions for Escapes in Regular Expressions
 =================================================
 
    Until this chapter, we have only encountered escapes of the form
 `\^', which tell `sed' not to interpret the circumflex as a special
 character, but rather to take it literally.  For example, `\*' matches
 a single asterisk rather than zero or more backslashes.
 
    This chapter introduces another kind of escape(1)--that is, escapes
 that are applied to a character or sequence of characters that
 ordinarily are taken literally, and that `sed' replaces with a special
 character.  This provides a way of encoding non-printable characters in
 patterns in a visible manner.  There is no restriction on the
 appearance of non-printing characters in a `sed' script but when a
 script is being prepared in the shell or by text editing, it is usually
 easier to use one of the following escape sequences than the binary
 character it represents:
 
    The list of these escapes is:
 
 `\a'
      Produces or matches a BEL character, that is an "alert" (ASCII 7).
 
 `\f'
      Produces or matches a form feed (ASCII 12).
 
 `\n'
      Produces or matches a newline (ASCII 10).
 
 `\r'
      Produces or matches a carriage return (ASCII 13).
 
 `\t'
      Produces or matches a horizontal tab (ASCII 9).
 
 `\v'
      Produces or matches a so called "vertical tab" (ASCII 11).
 
 `\cX'
      Produces or matches `CONTROL-X', where X is any character.  The
      precise effect of `\cX' is as follows: if X is a lower case
      letter, it is converted to upper case.  Then bit 6 of the
      character (hex 40) is inverted.  Thus `\cz' becomes hex 1A, but
      `\c{' becomes hex 3B, while `\c;' becomes hex 7B.
 
 `\dXXX'
      Produces or matches a character whose decimal ASCII value is XXX.
 
 `\oXXX'
      Produces or matches a character whose octal ASCII value is XXX.
 
 `\xXX'
      Produces or matches a character whose hexadecimal ASCII value is
      XX.
 
    `\b' (backspace) was omitted because of the conflict with the
 existing "word boundary" meaning.
 
    Other escapes match a particular character class and are valid only
 in regular expressions:
 
 `\w'
      Matches any "word" character.  A "word" character is any letter or
      digit or the underscore character.
 
 `\W'
      Matches any "non-word" character.
 
 `\b'
      Matches a word boundary; that is it matches if the character to
      the left is a "word" character and the character to the right is a
      "non-word" character, or vice-versa.
 
 `\B'
      Matches everywhere but on a word boundary; that is it matches if
      the character to the left and the character to the right are
      either both "word" characters or both "non-word" characters.
 
 `\`'
      Matches only at the start of pattern space.  This is different
      from `^' in multi-line mode.
 
 `\''
      Matches only at the end of pattern space.  This is different from
      `$' in multi-line mode.
 
 
    ---------- Footnotes ----------
 
    (1) All the escapes introduced here are GNU extensions, with the
 exception of `\n'.  In basic regular expression mode, setting
 `POSIXLY_CORRECT' disables them inside bracket expressions.
 

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