I really had big troubles understanding all the preg_ functions in pset 6, 7 and 8, especially its cryptic first argument which is called a regular expression and looks like this:
CS50 Stack Exchange is a question and answer site for students of Harvard University's CS50. It only takes a minute to sign up.Sign up to join this community
I hope I can save someone some headache:
Eventually this website helped me understand regular expressions and that they are a very powerful tool every programmer should know:
Read here only the first paragraph: http://www.regular-expressions.info/index.html then head directly to the Quick Start Section: http://www.regular-expressions.info/quickstart.html
It's worth to spend some minutes there. CS50 Staff should have add this links into the problem sets recommended reading list!