Program or website to monitor for changes in source code of a website

I'm looking for a way to monitor a webpage for changes in its source code. I'd want to get an alert whenever a specific string appears in its HTML.

So far, I've tried the Distill plugin for Firefox, but didn't seem to have a way to directly check the website's source.

I also found some various websites that could send you an alert whenever a webpage changes, but none of them had the feature to check for a specific string in the source.

Anyone know of a way I can do this?

Thank you!