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How to validate a domain name using Regular Expression
• Last Updated : 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:

1. The domain name should be a-z or A-Z or 0-9 and hyphen (-).
2. The domain name should be between 1 and 63 characters long.
3. The domain name should not start or end with a hyphen(-) (e.g. -geeksforgeeks.org or geeksforgeeks.org-).
4. The last TLD (Top level domain) must be at least two characters and a maximum of 6 characters.
5. 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.
• 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 ``#include ``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 = ``"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

 `// 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}(?

## 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}(?
Output:
```true
true
false
false
false```

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