MongoDB regex search: security regarding user input

If I can guarantee input ($value in below example) is string (ie. attacker can't inject using PHP magic array), is following code sufficient for preventing injection?

$regex = str_replace('%', '', $value);

if (substr($value, 0, 1) != '%') $regex = '^' . $regex;
if (substr($value, -1) != '%') $regex = $regex . '$';

$value = new MongoRegex("/$regex/i");

Generally speaking, is MongoRegex("/$user-input/i") ok in terms of MongoDB security? Or should we take more precaution as in SQL world?