regex issues

I believe the problem is that you're not specifically evaluating to the end of the string. You need to use the "$" character at the end to indicate "to end of string".

when HTTP_REQUEST {
     switch -regexp [string tolower [HTTP::uri]] {
        "/first/url/files/[0-9]{0,7}$" {
        log local0. "You passed URL inspection!"
    }
    default {
        reject
        log local0. "rejected"
    }
}

This will catch any numeric value from nothing to 1000000. Change the evaluation to {1,7} if you want to make sure there's at least one digit in the string.