While solving the NUM3RS problem of CS50P, I was asked to match IPv4 addresses with RegExp.

A valid IPv4 address should be like this:

$$ a.b.c.d:e $$

where

$$ a,b,c,d \in [0, 255] \ e \in [0, 65525] $$

RegExp doesn’t support the direct expression of a number range. We have to do it some other way.

The 0-255 Part

After some analysis, we can easily see that we can do the 0-255 part this way:

and the RegExp should be like this:

[0-9]|([1-9][0-9])|(1[0-9][0-9])|(2[0-4][0-9])|25[0-5]

or

\d|([1-9]\d)|(1\d\d)|(2[0-4]\d)|(25[0-5])

The 0-65525 Part

With similar method, we can write the 0-65535 part like this:

[0-9]|[1-9][0-9]{1,3}|[1-5][0-9]{4}|6[0-4][0-9]{3}|65[0-4][0-9]{2}|655[0-2][0-9]|6553[0-5]

Concatenate Them

^((([0-9]|([1-9][0-9])|(1[0-9][0-9])|(2[0-4][0-9])|25[0-5])\.){3}([0-9]|([1-9][0-9])|(1[0-9][0-9])|(2[0-4][0-9])|25[0-5]))((:([0-9]|[1-9][0-9]{1,3}|[1-5][0-9]{4}|6[0-4][0-9]{3}|65[0-4][0-9]{2}|655[0-2][0-9]|6553[0-5]))?)$