my $regex = qr/
(

123\.145\.14[6-9]\.2       # match first specified range

|                          # or

                           # second specified range:
135\.168\.                 # <= first 2 octets
(?:                        # 3rd octet:
1 (?: 0[0-9]|1[0-5] )      # 100 - 109 or 110 - 115
|                          # or
[1-9][0-9]                 # 10 - 99
)
\.                         # 4th octet:
(?:
(?:1[01][0-9]|12[0-5])     # 100 - 119 or 120 - 125
|                          # or
[0-9][0-9]?                # 0 - 99
)
$                          # end of string
)
/ox;