Iterators on iterators. Get all filenames in a directory (recursively) which matches a given regular expression. Iterators: RegexIterator, RegexIteratorIterator, RegexDirectoryIterator, RecursiveRegexIterator.

iterateOverFilenamesInDirectoryRecursivelyBasedOnRegex.php

$iterator = new \RegexIterator(
  new \RecursiveIteratorIterator(
      new \RecursiveDirectoryIterator($path, \FilesystemIterator::SKIP_DOTS),
      \RecursiveIteratorIterator::LEAVES_ONLY
  ),
  '%^.*?(([^\\\]+?)\.png)$%',
  \RecursiveRegexIterator::GET_MATCH
);

foreach ($iterator as $file) {
  
  // CAN access groups / references in regular expression used in RegexIterator :)
  print_r($file);
  /*
   * Example output:
   * array ( [0] => C:\directory\filename.png [1] => filename.png [2] => filename )
   */
  
}