How to validate an IP address using ReGex

Given an IP address, the task is to validate this IP address with the help of Regex (Regular Expression) in C++ as a valid IPv4 address or IPv6 address. If IP address is not valid then print invalid IP address.


Input: str = “”
Output: Valid IPv4

Input: str = “”
Output: Invalid IP

Input: str = “2F33:12a0:3Ea0:0302”
Output: Valid IPv6

Input: str = “”
Output: Invalid IP


  1. Regex (Regular Expression) In C++ will be used to check the IP address.
  2. Range Specifications
    Specifying a range of characters or literals is one of the simplest criteria used in a regex.

    i) [a-z]
    ii) [A-Za-z0-9]
    • In the above expression ([]) square brackets are used to specify the range.
    • The first expression will match exactly one lowercase character.
    • The second expression specifies the range containing one single uppercase character, one lowercase character, and a digit from 0 to 9.
  3. Now to include a ‘.’ as part of an expression, we need to escape ‘.’ and this can be done as :

    The above expression indicates an ‘.’ and a digit in the range 0 to 9 as a regex.

  4. regex_match() function is used to match the given pattern. This function returns true if the given expression matches the string. Otherwise, the function returns false.

Here is the implementation of the above approach.





// C++ program to validate
// IP address using Regex
#include <bits/stdc++.h>
using namespace std;
// Function for Validating IP
string Validate_It(string IP)
    // Regex expression for validating IPv4
    regex ipv4("(([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])");
    // Regex expression for validating IPv6
    regex ipv6("((([0-9a-fA-F]){1,4})\\:){7}([0-9a-fA-F]){1,4}");
    // Checking if it is a valid IPv4 addresses
    if (regex_match(IP, ipv4))
        return "Valid IPv4";
    // Checking if it is a valid IPv6 addresses
    else if (regex_match(IP, ipv6))
        return "Valid IPv6";
    // Return Invalid
    return "Invalid IP";
// Driver Code
int main()
    // IP addresses to validate
    string IP = "";
    cout << Validate_It(IP) << endl;
    IP = "fffe:3465:efab:23fe:2235:6565:aaab:0001";
    cout << Validate_It(IP) << endl;
    IP = "2F33:12a0:3Ea0:0302";
    cout << Validate_It(IP) << endl;
    return 0;



Valid IPv4
Valid IPv6
Invalid IP

Time Complexity : O (N)
Auxiliary Space : O (1)

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