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 a 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 an HTML document.

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

html

<h1 style="color:green;" id="h1">
    GeeksforGeeks
</h1>
<p id="GFG_UP">
</p>
<button onclick="GFG_Fun()">
    click here
</button>
<p id="GFG_DOWN">
</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>

Output:

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

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

html

<h1 style="color:green;" id="h1">
    GeeksforGeeks
</h1>
<p id="GFG_UP">
</p>
<button onclick="GFG_Fun()">
    click here
</button>
<p id="GFG_DOWN">
</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>

Output:

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