Assertions (available for both enhanced basic and enhanced extended REs)
     In addition to `^' and `$' (the assertions that match the null string at the
     beginning and end of line, respectively), the following assertions become
     available:

           \<  Matches the null string at the beginning of a word.  This is equiv-
               alent to `[[:<:]]'.

           \>  Matches the null string at the end of a word.  This is equivalent
               to `[[:>:]]'.

           \b  Matches the null string at a word boundary (either the beginning or

 -w, --word-regexp
         The expression is searched for as a word (as if surrounded by
         `[[:<:]]' and `[[:>:]]'; see re_format(7)).

     There are two special cases= of bracket expressions: the bracket expressions
     `[[:<:]]' and `[[:>:]]' match the null string at the beginning and end of a
     word respectively.  A word is defined as a sequence of word characters which
     is neither preceded nor followed by word characters.  A word character is an
     alnum character (as defined by ctype(3)) or an underscore.  This is an exten-
     sion, compatible with but not specified by IEEE Std 1003.2 (``POSIX.2''), and
     should be used with caution in software intended to be portable to other sys-

  1 (?:
  2     (?<= \w)  (?! \w )
  3   | (?<! \w)  (?= \w )
  4 )

  1 (?:            # EITHER THIS PAIR IS TRUE:
  2     (?<= \w )  #    yes word-char right before this
  3                # and also
  4     (?!  \w )  #    no  word-char right after this
  5   |            # OR ELSE THIS PAIR IS TRUE:
  6     (?<! \w )  #    no  word-char right before this
  7                # and also
  8     (?=  \w )  #    yes word-char right after this
  9 )