- Allow NULL for $matches argument (helps when using preg_match only for
  match condition) - might not be possible

- http://bugs.php.net/bug.php?id=36983
  http://bugs.php.net/bug.php?id=33151

- I'd love to see a pattern modifer which says "don't fill $matches except
  for the overall match and any specific named captures". This would allow
  (?: ...) to be avoided in a lot of cases.
  
  This could make for non-trivial speed enhancements with regexes that have
  a lot of parens when working on long strings, since you'd not have to  
  copy them multiple times to the $matches array.
  
  Also, it makes $matches much cleaner after a match where you've named the
  captures you're interested in.
  
  (Note that this would not involve the use of PCRE_NO_AUTO_CAPTURE, as
  that would change the semantics of backreferences)

- In looking at preg_grep, I think it'd be pretty easy to implement some flags:

   PREG_GREP_REKEY_ALL --- all key values are rekeyed, as if return array had been
                           passed through array_values().
  
   PREG_GREP_REKEY_NUMS --- only numeric key values are rekeyed; string key values
                            are maintained

   PREG_GREP_REKEY_NONE  ---- (default and current situation) keys are maintained
  
  I can't judge the social effect of changing the default, but my intuition
  of a grep function in PHP would be that the default is PREG_GREP_REKEY_NUMS
  or PREG_GREP_REKEY_ALL....