A demo to replace all matches by user selected value

JavaScript / HTML/ CSS Code

Output

<!DOCTYPE html>
<html>
<head>
<style>
.divclass {
  background: #3B505F;
  height: auto;
  width:350px;
  border-radius: 15px;
  padding:20px;
  font-size:16px;
  color:#fff;  
}
span {
    background: #F7FB4D;
    color:#000;
    font-size:16px;
    font-weight:700;
}
.btncls{
    background-color:#FC2209;
    border: #2e6da4;
  	font-family: Arial, Geneva, Arial, Helvetica,  sans-serif;
	font-size: 15px;
    color: #FFFFFF;
    letter-spacing: 1px;
    padding: 8px 12px;
    font-size: 14px;
    font-weight: normal;

    border-radius: 4px;
    line-height: 1.5;
    text-decoration:none;
} 
</style>
</head>

<body>
<div class="divclass" id="repdemo">This Is A Test Sentence With Titlecase testing variables. this is a test sentence with small letters to test variables. This Is A Test Sentence With Titlecase. this is a test sentence with small letters. 
</div>

<p><div class="divclass">
<p><label>Replace Term: </label><select id="repterm">
    <option value="test">test</option>
    <option value="Is">Is</option>
    <option value="Sentence">Sentence</option>
</select>
</p>
<p>
<label>To be Replaced with: </label><select id="reptoterm">
    <option value="TEST">TEST</option>
    <option value="Was">Was</option>
    <option value="DIV">DIV</option>
</select>
</p>
</div>  
<p><button class="btncls" onclick="replacetxt()">Execute replace method</button></p>

</p>
<script type="text/javascript">
var src_str = document.getElementById("repdemo").innerHTML;



function replacetxt(){
    var rep_term = document.getElementById("repterm").value;
    var rep_to = document.getElementById("reptoterm").value;
    var rep_regex = new RegExp(rep_term,"gi");
    var str_rep = src_str.replace(rep_regex,"<span>" + rep_to + "</span>");

    document.getElementById("repdemo").innerHTML = str_rep;

}

</script>
</body>
</html>

This Is A Test Sentence With Titlecase testing variables. this is a test sentence with small letters to test variables. This Is A Test Sentence With Titlecase. this is a test sentence with small letters.

Replace Term:

To be Replaced with: