I'm curious: I see a lot of posts here which use ereg instead of preg. preg seems to be more versatile to me, so I'm wondering why one would use ereg instead. Am I missing something?

I'm not sure really, I don't think it really matters. But I tend to use ereg if I'm just trying to test the value of something for a simple true or false eval, but use preg for matching in all other instances.

preg is better for complex uses, and is more standar regex use, but in some servers theres is no the library to use with php. In php5 is installed, but i not sure
I use preg when i can.

Preg functions are faster and better for complex patterns. Its also Perl compatible, if you are familiar with that language. I have made it a habit to always use preg functions. The syntax is not very hard to learn when compared to ereg, so I don't find any reason why to use ereg, unless your just starting out with regular expressions. Even so, its probably better to just forget learning ereg in the first place.

Preg also has the function preg_match_all() which is extremely useful, and has no counterpart in the ereg function group.