(PHP 5 >= 5.2.0)
This iterator can be used to filter another iterator based on a regular expression.
$iterator
   , string $regex
   [, int $mode = self::MATCH
   [, int $flags = 0
   [, int $preg_flags = 0
  ]]] )RegexIterator::ALL_MATCHES
      Return all matches for the current entry (see preg_match_all()).
RegexIterator::GET_MATCH
      Return the first match for the current entry (see preg_match()).
RegexIterator::MATCH
      Only execute match (filter) for the current entry (see preg_match()).
RegexIterator::REPLACE
      Replace the current entry (see preg_replace(); Not fully implemented yet)
RegexIterator::SPLIT
      Returns the split values for the current entry (see preg_split()).
RegexIterator::USE_KEY
      Special flag: Match the entry key instead of the entry value.