How to check if a string is html or not using JavaScript?

Difficulty Level : Easy
Last Updated : 31 Jul, 2019

The task is to validate whether the given string is valid HTML or not using JavaScript. we’re going to discuss few techniques.

Approach

Get the HTML string into a variable.
Create a RegExp which checks for the validation.
RegExp should follow the rules of creating a HTML document.

Example 1: In this example, a regexp is created and it is validating the HTML string as valid.

<!DOCTYPE HTML> 
<html> 
  
<head> 
    <title> 
        JavaScript  
      | Check if a string is html or not. 
    </title> 
</head> 
  
<body style="text-align:center;" 
      id="body"> 
    <h1 style="color:green;" 
        id="h1">   
            GeeksForGeeks   
        </h1> 
    <p id="GFG_UP" 
       style="font-size: 15px; 
              font-weight: bold;"> 
    </p> 
    <button onclick="GFG_Fun()"> 
        click here 
    </button> 
    <p id="GFG_DOWN"
       style="color:green; 
              font-size: 20px; 
              font-weight: bold;"> 
    </p> 
    <script> 
        var up = document.getElementById('GFG_UP'); 
        var str1 = '<div>GeeksForGeeks</div>'; 
        var str = '<div>GeeksForGeeks</div>'; 
        
        up.innerHTML = "Click on the button to check "+ 
          "for the valid HTML.<br> String - " + str1; 
        
        var down = document.getElementById('GFG_DOWN'); 
  
        function GFG_Fun() { 
            down.innerHTML =  
/<(?=.*? .*?\/ ?>|br|hr|input|!--|wbr)[a-z]+.*?>|<([a-z]+).*?<\/\1>/i.test(str); 
        } 
    </script> 
</body> 
  
</html> 

Output:

Before clicking on the button:
After clicking on the button:

Example 2: In this example, Here also, A regexp is created and it is validating the HTML string as invalid.

<!DOCTYPE HTML> 
<html> 
  
<head> 
    <title> 
        JavaScript  
      | Check if a string is html or not. 
    </title> 
</head> 
  
<body style="text-align:center;" 
      id="body"> 
    <h1 style="color:green;" 
        id="h1">   
            GeeksForGeeks   
        </h1> 
    <p id="GFG_UP" 
       style="font-size: 15px; 
              font-weight: bold;"> 
    </p> 
    <button onclick="GFG_Fun()"> 
        click here 
    </button> 
    <p id="GFG_DOWN"
       style="color:green; 
              font-size: 20px; 
              font-weight: bold;"> 
    </p> 
    <script> 
        var up = document.getElementById('GFG_UP'); 
        var str1 = '<div>GeeksForGeeks</dv>'; 
        var str = '<div>GeeksForGeeks</dv>'; 
        
        up.innerHTML = "Click on the button to check "+ 
          "for the valid HTML.<br> String - " + str1; 
        
        var down = document.getElementById('GFG_DOWN'); 
  
        function GFG_Fun() { 
            down.innerHTML =  
 /<([A-Za-z][A-Za-z0-9]*)\b[^>]*>(.*?)<\/\1>/.test(str); 
        } 
    </script> 
</body> 
  
</html> 

Output:

Before clicking on the button:
After clicking on the button:

My Personal Notes

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