You might try this:
<?php
$str="This is a very large number 999,999,999.999 !";
echo $str = preg_replace( '/[^\d,.]/', '', $str );
to only allow:
- digits (0-9)
- commas (,)
- dots (.)
You might then consider filters like:
add_{$meta_type}_metadata
update_{$meta_type}_metadata
get_{$meta_type}_metadata
where $meta_type is the type of object metadata (comment, post or user).
ps: just remember to take backup before testing these filters 😉