How to validate HTML tag using Regular Expression

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

1. It should start with an opening tag (<).
2. It should be followed by a double quotes string or single quotes string.
3. It should not allow one double quotes string, one single quotes string or a closing tag (>) without single or double quotes enclosed.
4. It should end with a closing tag (>).

Examples:

Input: str = “<input value = ‘>’>”; 
Output: true 
Explanation: The given string satisfies all the above mentioned conditions.
Input: str = “<br/>”; 
Output: true 
Explanation: The given string satisfies all the above mentioned conditions.
Input: str = “br/>”; 
Output: false 
Explanation: The given string doesn’t starts with an opening tag “<“. Therefore, it is not a valid HTML tag.
Input: str = “<‘br/>”; 
Output: false 
Explanation: The given string has one single quotes string that is not allowed. Therefore, it is not a valid HTML tag.
Input: str = “<input value => >”; 
Output: false 
Explanation: The given string has a closing tag (>) without single or double quotes enclosed that is not allowed. Therefore, it is not a valid HTML tag.

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 valid HTML tag as mentioned below: 
  

regex = “<(“[^”]*”|'[^’]*’|[^'”>])*>”; 

• Where: 
  • < represents the string should start with an opening tag (<).
  • ( represents the starting of the group.
  • “[^”]*” represents the string should allow double quotes enclosed string.
  • | represents or.
  • ‘[^’]*‘ represents the string should allow single quotes enclosed string.
  • | represents or.
  • [^'”>] represents the string should not contain one single quote, double quotes, and “>”.
  • ) represents the ending of the group.
  • * represents 0 or more.
  • > represents the string should end with a closing tag (>).
• 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
// HTML tag using Regular Expression
#include <iostream>
#include <regex>
using namespace std;

// Function to validate the HTML tag.
bool isValidHTMLTag(string str)
{

  // Regex to check valid HTML tag.
  const regex pattern("<(\"[^\"]*\"|'[^']*'|[^'\">])*>");

  // If the HTML tag
  // is empty return false
  if (str.empty())
  {
     return false;
  }

  // Return true if the HTML tag
  // matched the ReGex
  if(regex_match(str, pattern))
  {
    return true;
  }
  else
  {
    return false;
  }
}

// Driver Code
int main()
{

  // Test Case 1:
  string str1 = "<input value = '>'>";
  cout << isValidHTMLTag(str1) << endl;

  // Test Case 2:
  string str2 = "<br/>";
  cout << isValidHTMLTag(str2) << endl;

  // Test Case 3:
  string str3 = "br/>";
  cout << isValidHTMLTag(str3) << endl;

  // Test Case 4:
  string str4 = "<'br/>";
  cout << isValidHTMLTag(str4) << endl;

  // Test Case 5:
  string str5 = "<input value => >";
  cout << isValidHTMLTag(str5) << endl;

  return 0;
}

// This code is contributed by yuvraj_chandra

Java

// Java program to validate
// HTML tag using regex.

import java.util.regex.*;

class GFG {

    // Function to validate
    // HTML tag using regex.
    public static boolean
    isValidHTMLTag(String str)
    {
        // Regex to check valid HTML tag.
        String regex
            = "<(\"[^\"]*\"|'[^']*'|[^'\">])*>";

        // Compile the ReGex
        Pattern p = Pattern.compile(regex);

        // If the string is empty
        // return false
        if (str == null) {
            return false;
        }

        // Find match between given string
        // and regular expression
        // using Pattern.matcher()
        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 = "<input value = '>'>";
        System.out.println(isValidHTMLTag(str1));

        // Test Case 2:
        String str2 = "<br/>";
        System.out.println(isValidHTMLTag(str2));

        // Test Case 3:
        String str3 = "br/>";
        System.out.println(isValidHTMLTag(str3));

        // Test Case 4:
        String str4 = "<'br/>";
        System.out.println(isValidHTMLTag(str4));

        // Test Case 5:
        String str5 = "<input value => >";
        System.out.println(isValidHTMLTag(str5));
    }
}

Python3

# Python3 program to validate
# HTML tag using regex.
# using regular expression
import re

# Function to validate
# HTML tag using regex.
def isValidHTMLTag(str):

    # Regex to check valid
    # HTML tag using regex.
    regex = "<(\"[^\"]*\"|'[^']*'|[^'\">])*>"

    # 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 = "<input value = '>'>"
print(isValidHTMLTag(str1))

# Test Case 2:
str2 = "<br/>"
print(isValidHTMLTag(str2))

# Test Case 3:
str3 = "br/>"
print(isValidHTMLTag(str3))

# Test Case 4:
str4 = "<'br/>"
print(isValidHTMLTag(str4))

# This code is contributed by avanitrachhadiya2155

Output:

true
true
false
false
false