# How to validate a domain name using Regular Expression

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. write.geeksforgeeks.org).

Examples:

Input: str = “write.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 = ``"write.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}(?

## 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={``"geeksforgeeks.org"``,``"write.geeksforgeeks.org"``,``"-geeksforgeeks.org"``,``"geeksforgeeks.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}(?

## 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 = ``"geeksforgeeks.org"``;` `console.log(isValidDomain(str1));`   `// Test Case 2:` `let str2 = ``"contribute.geeksforgeeks.org"``;` `console.log(isValidDomain(str2));`   `// Test Case 3:` `let str3 = ``"-geeksforgeeks.org"``;` `console.log(isValidDomain(str3));`   `// Test Case 4:` `let str4 = ``"geeksforgeeks.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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