html

<section>
    <textarea placeholder="Enter Your Text" id="user_text" cols="30" rows="5" style="margin-left: -10px;"></textarea>
    <br>
    <input type="text" placeholder="What You Want To Search..." id="search" style="width: 307px;margin-left: -10px;">
    <br>
    <button type="submit" onclick="handle_search()">Search</button>
    <span style="color: #b5e853;" id="output"></span>
</section>
        

js

<script>
    let user_text = document.getElementById('user_text')
    let search_query = document.getElementById('search')
    let output = document.getElementById('output')
    const handle_search = () => {
        if(user_text.value.length > 0 && search_query.value.length > 0){
            let regex_query = new RegExp(search_query.value, "ig");
            if(regex_query.test(user_text.value)){
                output.innerHTML = "Exist"
            }else{
                output.innerHTML = "Not Exist"
            }
        }else{
            return false
        }
    }
</script>