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JavaScript

let str = "3 HTML tags are: <html>, <a href src='www.google.com'>, and <body>.";
let map = {
   "<html>":"<HTML>",
   "<a*>":"<A HREF SRC='..'",
   "<body>":"<BODY>"
};
str = str.replace(/<html>|<a*>|<body>/gi, function(matched){
  return map[matched];
});

alert(str);