Log Message:
Changes 2.19:
** Improvements

  Performance has improved, typically by 10% and in some cases by a
  factor of 200.  However, performance of grep -P in UTF-8 locales has
  gotten worse as part of the fix for the crashes mentioned below.

** Bug fixes

  grep no longer mishandles patterns like [a-[.z.]], and no longer
  mishandles patterns like [^a] in locales that have multicharacter
  collating sequences so that [^a] can match a string of two characters.

  grep no longer mishandles an empty pattern at the end of a pattern list.
  [bug introduced in grep-2.5]

  grep -C NUM now outputs separators consistently even when NUM is zero,
  and similarly for grep -A NUM and grep -B NUM.
  [bug present since "the beginning"]

  grep -f no longer mishandles patterns containing NUL bytes.
  [bug introduced in grep-2.11]

  Plain grep, grep -E, and grep -F now treat encoding errors in patterns
  the same way the GNU regular expression matcher treats them, with respect
  to whether the errors can match parts of multibyte characters in data.
  [bug present since "the beginning"]

  grep -w no longer mishandles a potential match adjacent to a letter that
  takes up two or more bytes in a multibyte encoding.
  Similarly, the patterns '\<', '\>', '\b', and '\B' no longer
  mishandle word-boundary matches in multibyte locales.
  [bug present since "the beginning"]

  grep -P now reports an error and exits when given invalid UTF-8 data.
  Previously it was unreliable, and sometimes crashed or looped.
  [bug introduced in grep-2.16]

  grep -P now works with -w and -x and backreferences. Before,
  echo aa|grep -Pw '(.)\1' would fail to match, yet
  echo aa|grep -Pw '(.)\2' would match.

  grep -Pw now works like grep -w in that the matched string has to be
  preceded and followed by non-word components or the beginning and end
  of the line (as opposed to word boundaries before).  Before, this
  echo a@@a| grep -Pw @@ would match, yet this
  echo a@@a| grep -w @@ would not.  Now, they both fail to match,
  per the documentation on how grep's -w works.

  grep -i no longer mishandles patterns containing titlecase characters.
  For example, in a locale containing the titlecase character
  'Lj' (U+01C8 LATIN CAPITAL LETTER L WITH SMALL LETTER J),
  'grep -i Lj' now matches both 'LJ' (U+01C7 LATIN CAPITAL LETTER LJ)
  and 'lj' (U+01C9 LATIN SMALL LETTER LJ).