Regular Expression (IP and domain name)

 

1.

A complete domain name consists of the root domain, top-level domain, level 2, level 3 ...... Domain names are separated by dots. Each domain name consists of letters, numbers, and minus signs (the first letter cannot be a minus sign). It is case-insensitive and cannot exceed 63 characters in length.
Obviously, a separate name can be matched by a regular expression [a-zA-Z0-9] [-a-zA-Z0-9] {}, and a complete domain name contains at least two names (such as google.com, it is composed of google and com. At last, there can be a point that represents the root domain (in the specification, the last point is the complete domain name, however, it is generally considered that a domain name with more than two names is also a complete domain name, even if it is not followed ).
The regular expression that matches the complete Domain Name:
[A-zA-Z0-9] [-a-zA-Z0-9] {} (\. [a-zA-Z0-9] [-a-zA-Z0-9] {}) + \.?

 

2.

An IP string consisting of four segments, each of which is 0 ~ A decimal point. For example, 61.139.2.69 is a legal IP string.
If the regular expression is written as \ d {1, 3} (\. \ d {1, 3}) {3}, It is not responsible because it can match an invalid IP string such as 300.400.555.666.
To match a 0 ~ There are several matching methods for the number between 255:
Regular Expression matching description
0 ~ 9 \ d single digit
10 ~ 99 [1-9] \ d two digits
100 ~ 199 1 \ d the number of three digits with one hundred bits
200 ~ 249 2 [0-4] \ d three-digit number, bits are 2, and 10 bits are 0 ~ 9
250 ~ 255 25 [0-5] three digits, digits are 2, 10 digits are 5, and one digit is 0 ~ 5
Written as a regular expression, that is, (\ d | ([1-9] \ d) | (1 \ d) | (2 [0-4] \ d) | (25 [0-5]), but when such a regular expression matches strings like 254, it will match 2, 5, and 4 respectively to get three matches, if the expected results are not met, the correct method is to reverse the order to (25 [0-5]) | (2 [0-4] \ d) | (1 \ d) | ([1-9] \ d) | \ d), because in the (xxx | yyy) Matching Behavior, It is searched from left to right.
The complete regular expression is:
(25 [0-5]) | (2 [0-4] \ d) | (1 \ d) | ([1-9] \ d) | \ d )(\. (25 [0-5]) | (2 [0-4] \ d) | (1 \ d) | ([1-9] \ d) | \ d) {3}
Press:
A number with a high value of 0 such as 061 cannot be matched.

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Therefore, the top (1. Part) method is the correct full version, and the (2. Part) method is one-sided.