better understanding of ereg_replace and other functions

Hi everybody, Lot of places in manual i came across using "ereg_replace" or "ereg" is very resource intensive and hence its better to use "preg_replace" or "str_replace" or "strpos" if they do the jobs. Now i am very curious to look at the source code (implementation) of these function so as to be able to decide the right choice for me. Can any one of you tell me where to find the source code of these functions. Do they require special privileges ?? Please help me out Binay