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

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.

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<!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>

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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.

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<!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>

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Output:

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

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