It can be made with JavaScript with function replace():
function onTextareaInput() {
document.getElementById("textarea").value = document.getElementById("textarea").value.replace("hc", "hollycrap");
}<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>Testtt</title>
</head>
<body>
<textarea id="textarea" oninput="onTextareaInput()"></textarea>
</body>
</html>