native.php

<?php

// Let's get a list of Post IDs that have given post meta item matching a pattern, but we only want unique values
// And we want the items ordered correctly so that keys aren't skipping numbers, etc.
$post_ids = array_values( array_unique( wp_list_pluck( array_filter( (new WP_Query(['posts_per_page' => -1]))->posts, function($post) {
  return (bool) preg_match( '/some-pattern/i', $post->some_meta_value );
} ), 'ID' ) );

with-collections.php

<?php

// Now compare this to using collections
$post_ids = collect( (new WP_Query(['posts_per_page' => -1]))->posts )
  ->filter( function($post) { return (bool) preg_match( '/some-pattern/i', $post->some_meta_value ); } )
  ->pluck( 'ID' )
  ->unique()
  ->values()
  ->all();