This one's pretty nice.  After getting frustrated for hunting down .jpg files in my massive music collection (PHP would run out of memory), I thought there should be a preg_ls function.

function preg_ls ($path=".", $rec=false, $pat="/.*/") {
    // it's going to be used repeatedly, ensure we compile it for speed.
    $pat=preg_replace("|(/.*/[^S]*)|s", "\\1S", $pat);
    //Remove trailing slashes from path
    while (substr($path,-1,1)=="/") $path=substr($path,0,-1);
    //also, make sure that $path is a directory and repair any screwups
    if (!is_dir($path)) $path=dirname($path);
    //assert either truth or falsehoold of $rec, allow no scalars to mean truth
    if ($rec!==true) $rec=false;
    //get a directory handle
    $d=dir($path);
    //initialise the output array
    $ret=Array();
    //loop, reading until there's no more to read
    while (false!==($e=$d->read())) {
        //Ignore parent- and self-links
        if (($e==".")||($e=="..")) continue;
        //If we're working recursively and it's a directory, grab and merge
        if ($rec && is_dir($path."/".$e)) {
            $ret=array_merge($ret,preg_ls($path."/".$e,$rec,$pat));
            continue;
        }
        //If it don't match, exclude it
        if (!preg_match($pat,$e)) continue;
        //In all other cases, add it to the output array
        $ret[]=$path."/".$e;
    }
    //finally, return the array
    return $ret;
}

Not bad for a mere 18 lines, don't you think?

Example use:

foreach (preg_ls("/etc/X11", true, "/.*\.conf/i") as $file) echo $file."\n";

Output: 

/etc/X11/xkb/README.config
/etc/X11/xorg.conf-vesa
/etc/X11/xorg.conf~
/etc/X11/gui.conf
/etc/X11/xorg.conf
/etc/X11/xorg.conf-fbdev