The ereg functions support the POSIX regular expression standard. The preg functions are based on the regex syntax and functions used in the Perl language. The preg functions support a few more options -- as to whether it's faster than ereg, I don't know about that. I use the preg functions mainly because I learned Perl before I learned PHP.
"Please give us a simple answer, so that we don't have to think, because if we think, we might find answers that don't fit the way we want the world to be."
~ Terry Pratchett in Nation