Too iRule???

Or... If you wanted to step on the wild side, you could play some fun and games with string manipulation.

Assuming your uri always starts with "/Custxx" where xx is a number, then Something like this would be much more optimal.

when HTTP_REQUEST {
   Parsing uri of the format /Custxx/path
  set error 1
  if { [HTTP::uri] starts_with "/Cust" } {
     Set ranges for old and new uris
    set LOW_RANGE_OLD 1
    set HIGH_RANGE_OLD 57
    set LOW_RANGE_NEW 58
    set HIGH_RANGE_NEW 58
     extract characters 5 to end from uri
     ie. xx/path
    set cust_num [string range [HTTP::uri] 5 end]
     Search for possible slash separator in uri
    set slash [string first "/" $cust_num]
    if { -1 != $slash } {
       Slash found so extract all characters up until the slash
      incr slash -1
      set cust_num [string range $cust_num 0 $slash]
    }
     Account for the case of the uri formatted as "/Cust"
    if { "" ne $cust_num } {
       Search range of old customer numbers
       New customer numbers, and customer singles
      if { ($cust_num >= $LOW_RANGE_OLD) && ($cust_num <= $HIGH_RANGE_OLD) } {
        set error 0
        pool Pool1
      } elseif { ($cust_num >= $LOW_RANGE_NEW) && ($cust_num <= $HIGH_RANGE_NEW) } {
        set error 0
        pool Pool2
      } elseif { $cust_num == 59 } {
        set error 0
        HTTP::redirect https://www.somewebsite.com/
      }
    }
  }
  if { 1 == $error } {
    HTTP::redirect http://www.somewebsite.com/Error.html
  }
}

*Disclaimer, this rule has not been tested. Also, moving forward, since it was unclear from your initial post what you do with the customer numbers when they move forward you may either need to special cast them (if they go to different uris) or group them in a range.

If you special case them, then I'd recommend putting them in a string data group and using the findclass method to extract the target paths.

Good luck!

-Joe