Here's a really basic way of doing it with steemjs and nodejs.
var steem = require("steem")
steem.api.streamTransactions("irreversible", function(err, result) {
var op = result.operations[0]
if (op[0] == "comment"){
console.log(op)
if (op[1].body.includes("STUFFYOUARELOOKINGFOR")){
//DOSTUFF
}
}
})