This is about the IPv4 address format, a total length of 32 * 8 = 4 segments, with each segment between the segmentation, each segment decimal values are 0-255.
The regular expression represented 0-255, can be divided into what few are considered: 
the IP address format can be expressed as:. XXX.XXX.XXX.XXX, XXX in the range 0-255, the first three segments plus a repeat three times in the last paragraph of the merger and to form a complete format of an IP address.
Therefore, the IP address of said positive following method:
((25[0-5]|2[0-4]\d|((1\d{2})|([1-9]?\d))).){3}(25[0-5]|2[0-4]\d|((1\d{2})|([1-9]?\d)))
