Add a hook to usercp.php

[Wildcard]

Then just use:

$plugins->add_hook("usercp_do_profile_start", "my_function");

function my_function()
{
global $mybb;
// . . .
if (preg_match($regex, $mybb->input['profile_fields']['fid4'])) {
// filter here
}
}

You'll need to replace 'fid4' with 'fidX' X being the id of your new custom profile fields.