answer to gwlester

proc t srvurl {
    set validityRE {(?xi)
        ^
        # Path part (already must start with / character)
        (?:      [-\w.~!$&'()*+,;=:@/]  | %[0-9a-f][0-9a-f] )*
        # Query part (optional, permits ? characters)
        (?: \? (?: [-\w.~!$&'()*+,;=:@/?] | %[0-9a-f][0-9a-f] )* )?
        $
    }
    if {![regexp -- $validityRE $srvurl]} {
        # Provide a better error message in this error case
        if {[regexp {(?i)%(?![0-9a-f][0-9a-f])..} $srvurl bad]} {
            puts stdout "Yes!!  That is it!!"
        }
        puts stdout error "No, try again. :("
    }
}
% t http://foo.bar/snafu?%##
Yes!!  That is it!!
wrong # args: should be "puts ?-nonewline? ?channelId? string"
% t /snafu?%##
Yes!!  That is it!!
wrong # args: should be "puts ?-nonewline? ?channelId? string"
% t ?%##
Yes!!  That is it!!
wrong # args: should be "puts ?-nonewline? ?channelId? string"
% t %##
Yes!!  That is it!!
wrong # args: should be "puts ?-nonewline? ?channelId? string"