How to validate a domain name using Regular Expression

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Given string str, the task is to check whether the given string is a valid domain name or not by using Regular Expression.
The valid domain name must satisfy the following conditions:

The domain name should be a-z or A-Z or 0-9 and hyphen (-).
The domain name should be between 1 and 63 characters long.
The domain name should not start or end with a hyphen(-) (e.g. -zambiatek.com or zambiatek.com-).
The last TLD (Top level domain) must be at least two characters and a maximum of 6 characters.
The domain name can be a subdomain (e.g. write.zambiatek.com).

Examples:

Input: str = “write.zambiatek.com”
Output: true
Explanation:
The given string satisfies all the above mentioned conditions. Therefore, it is a valid domain name.
Input: str = “-zambiatek.com”
Output: false
Explanation:
The given string starts with a hyphen (-). Therefore, it is not a valid domain name.
Input: str = “zambiatek.o”
Output: false
Explanation:
The given string have last TLD of 1 character, the last TLD must be between 2 and 6 characters long. Therefore, it is not a valid domain name.
Input: str = “.org”
Output: false
Explanation:
The given string doesn’t start with a-z or A-Z or 0-9. Therefore, it is not a valid domain name.

Approach: The idea is to use Regular Expression to solve this problem. The following steps can be followed to compute the answer:

Get the String.
Create a regular expression to check the valid domain name as mentioned below:

regex = “^((?!-)[A-Za-z0-9-]{1, 63}(?<!-)\\.)+[A-Za-z]{2, 6}$”

Where:
- ^ represents the starting of the string.
- ( represents the starting of the group.
- (?!-) represents the string should not start with a hyphen (-).
- [A-Za-z0-9-]{1, 63} represents the domain name should be a-z or A-Z or 0-9 and hyphen (-) between 1 and 63 characters long.
- (?<!-) represents the string should not end with a hyphen (-).
- \\. represents the string followed by a dot.
- )+ represents the ending of the group, this group must appear at least 1 time, but allowed multiple times for subdomain.
- [A-Za-z]{2, 6} represents the TLD must be A-Z or a-z between 2 and 6 characters long.
- $ represents the ending of the string.
Match the given string with the regular expression. In Java, this can be done by using Pattern.matcher().
Return true if the string matches with the given regular expression, else return false.

Below is the implementation of the above approach:

C++

// C++ program to validate the
// domain name using Regular Expression
#include <iostream>
#include <regex>
using namespace std;
 
// Function to validate the domain name.
bool isValidDomain(string str)
{
 
  // Regex to check valid domain name.
  const regex pattern("^(?!-)[A-Za-z0-9-]+([\\-\\.]{1}[a-z0-9]+)*\\.[A-Za-z]{2,6}$");
 
  // If the domain name
  // is empty return false
  if (str.empty())
  {
     return false;
  }
 
  // Return true if the domain name
  // matched the ReGex
  if(regex_match(str, pattern))
  {
    return true;
  }
  else
  {
    return false;
  }
}
 
// Driver Code
int main()
{
   
  // Test Case 1:
  string str1 = "zambiatek.com";
  cout << isValidDomain(str1) << endl;
 
  // Test Case 2:
  string str2 = "write.zambiatek.com";
  cout << isValidDomain(str2) << endl;
 
  // Test Case 3:
  string str3 = "-zambiatek.com";
  cout << isValidDomain(str3) << endl;
 
  // Test Case 4:
  string str4 = "zambiatek.o";
  cout << isValidDomain(str4) << endl;
 
  // Test Case 5:
  string str5 = ".org";
  cout << isValidDomain(str5) << endl;
 
  return 0;
}
 
// This code is contributed by yuvraj_chandra

Java

// Java program to validate domain name.
// using regular expression.
 
import java.util.regex.*;
class GFG {
 
    // Function to validate domain name.
    public static boolean isValidDomain(String str)
    {
        // Regex to check valid domain name.
        String regex = "^((?!-)[A-Za-z0-9-]"
                       + "{1,63}(?<!-)\\.)"
                       + "+[A-Za-z]{2,6}";
 
        // Compile the ReGex
        Pattern p
            = Pattern.compile(regex);
 
        // If the string is empty
        // return false
        if (str == null) {
            return false;
        }
 
        // Pattern class contains matcher()
        // method to find the matching
        // between the given string and
        // regular expression.
        Matcher m = p.matcher(str);
 
        // Return if the string
        // matched the ReGex
        return m.matches();
    }
 
    // Driver Code
    public static void main(String args[])
    {
 
        // Test Case 1:
        String str1 = "zambiatek.com";
        System.out.println(isValidDomain(str1));
 
        // Test Case 2:
        String str2 = "write.zambiatek.com";
        System.out.println(isValidDomain(str2));
 
        // Test Case 3:
        String str3 = "-zambiatek.com";
        System.out.println(isValidDomain(str3));
 
        // Test Case 4:
        String str4 = "zambiatek.o";
        System.out.println(isValidDomain(str4));
 
        // Test Case 5:
        String str5 = ".org";
        System.out.println(isValidDomain(str5));
    }
}

Python3

# Python3 program to validate
# domain name
# using regular expression
import re
 
# Function to validate
# domain name.
def isValidDomain(str):
 
    # Regex to check valid
    # domain name.
    regex = "^((?!-)[A-Za-z0-9-]" + "{1,63}(?<!-)\\.)" +"+[A-Za-z]{2,6}"
     
    # Compile the ReGex
    p = re.compile(regex)
 
    # If the string is empty
    # return false
    if (str == None):
        return False
 
    # Return if the string
    # matched the ReGex
    if(re.search(p, str)):
        return True
    else:
        return False
 
# Driver code
 
# Test Case 1:
str1 = "zambiatek.com"
print(isValidDomain(str1))
 
# Test Case 2:
str2 = "write.zambiatek.com"
print(isValidDomain(str2))
 
# Test Case 3:
str3 = "-zambiatek.com"
print(isValidDomain(str3))
 
# Test Case 4:
str4 = "zambiatek.o"
print(isValidDomain(str4))
 
# Test Case 5:
str5 = ".org"
print(isValidDomain(str5))

C#

// C# program to validate domain name.
// using Regular Expressions
using System;
using System.Text.RegularExpressions;
class GFG
{
 
  // Main Method
  static void Main(string[] args)
  {
 
    // Input strings to Match
    // Valid Domain Name
    string[] str={"zambiatek.com","write.zambiatek.com","-zambiatek.com","zambiatek.o",
                  ".org"};
    foreach(string s in str) {
      Console.WriteLine( isValidDomain(s) ? "true" : "false"); 
    }
    Console.ReadKey(); }
 
  // method containing the regex
  public static bool isValidDomain(string str)
  {
    string strRegex = @"^((?!-)[A-Za-z0-9-]{1,63}(?<!-)\.)+[A-Za-z]{2,6}$";
    Regex re = new Regex(strRegex);
    if (re.IsMatch(str))
      return (true);
    else
      return (false);
  }
}
 
// This code is contributed by Rahul Chauhan

Javascript

// Javascript program to validate
// domain name using Regular Expression
 
// Function to validate the
// domain name
function isValidDomain(str) {
    // Regex to check valid
    // Domain Name
    let regex = new RegExp(/^(?!-)[A-Za-z0-9-]+([\-\.]{1}[a-z0-9]+)*\.[A-Za-z]{2,6}$/);
 
    // if str
    // is empty return false
    if (str == null) {
        return "false";
    }
 
    // Return true if the str
    // matched the ReGex
    if (regex.test(str) == true) {
        return "true";
    }
    else {
        return "false";
    }
}
 
// Driver Code
// Test Case 1:
let str1 = "zambiatek.com";
console.log(isValidDomain(str1));
 
// Test Case 2:
let str2 = "contribute.zambiatek.com";
console.log(isValidDomain(str2));
 
// Test Case 3:
let str3 = "-zambiatek.com";
console.log(isValidDomain(str3));
 
// Test Case 4:
let str4 = "zambiatek.o";
console.log(isValidDomain(str4));
 
// Test Case 5:
let str5 = ".org";
console.log(isValidDomain(str5));
 
// This code is contributed by Rahul Chauhan

Output:

true
true
false
false
false

Time Complexity: O(N) for each testcase, where N is the length of the given string.
Auxiliary Space: O(1)

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