How to validate a domain name using Regular Expression - GeeksforGeeks

04 Feb, 2021
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. -geeksforgeeks.org or geeksforgeeks.org-).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. contribute.geeksforgeeks.org).
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. -geeksforgeeks.org or geeksforgeeks.org-).
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. contribute.geeksforgeeks.org).
Examples: 
Input: str = “contribute.geeksforgeeks.org” Output: true Explanation: The given string satisfies all the above mentioned conditions. Therefore, it is a valid domain name.Input: str = “-geeksforgeeks.org” Output: false Explanation: The given string starts with a hyphen (-). Therefore, it is not a valid domain name.Input: str = “geeksforgeeks.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.
^ 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++
Java
Python3
// 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 Codeint main(){     // Test Case 1:  string str1 = "geeksforgeeks.org";  cout << isValidDomain(str1) << endl;   // Test Case 2:  string str2 = "contribute.geeksforgeeks.org";  cout << isValidDomain(str2) << endl;   // Test Case 3:  string str3 = "-geeksforgeeks.org";  cout << isValidDomain(str3) << endl;   // Test Case 4:  string str4 = "geeksforgeeks.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 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 = "geeksforgeeks.org";        System.out.println(isValidDomain(str1));         // Test Case 2:        String str2 = "contribute.geeksforgeeks.org";        System.out.println(isValidDomain(str2));         // Test Case 3:        String str3 = "-geeksforgeeks.org";        System.out.println(isValidDomain(str3));         // Test Case 4:        String str4 = "geeksforgeeks.o";        System.out.println(isValidDomain(str4));         // Test Case 5:        String str5 = ".org";        System.out.println(isValidDomain(str5));    }}
# Python3 program to validate# domain name# using regular expressionimport 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 = "geeksforgeeks.org"print(isValidDomain(str1)) # Test Case 2:str2 = "contribute.geeksforgeeks.org"print(isValidDomain(str2)) # Test Case 3:str3 = "-geeksforgeeks.org"print(isValidDomain(str3)) # Test Case 4:str4 = "geeksforgeeks.o"print(isValidDomain(str4)) # Test Case 5:str5 = ".org"print(isValidDomain(str5)) # This code is contributed by avanitrachhadiya2155
true
true
false
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avanitrachhadiya2155
yuvraj_chandra
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java-regular-expression
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