preg_grep_keys() allows you to grep over the keys of an array.

preg_grep_keys.php

<?php

function preg_grep_keys($pattern, $input) {
    return preg_grep($pattern, array_keys($input));
}

Did this really need a function though?

of course it did

This is the best one.

Better version available in the comments at http://php.net/manual/en/function.preg-grep.php (includes flags, this doesn't support inverse).

function preg_grep_keys( $pattern, $input, $flags = 0 )
{
    $keys = preg_grep( $pattern, array_keys( $input ), $flags );
    $vals = array();
    foreach ( $keys as $key )
    {
        $vals[$key] = $input[$key];
    }
    return $vals;
}

10/10 confirmed best function; using for production scale application