Unless you really need some advanced regular expressions only supported by PCRE, using POSIX regular expressions with grep is usually an order of magnitude faster – that’s because the default engine with grep uses finite automata, as opposed to a backtracking algorithm which PCRE uses ( the main featuress you gain from the backtracking algorithm are lookahead/lookbehind and backreferences)

Here’s a small example


$ time grep -E 'post:content.*facebook' a_bunch_of_files* | wc -l
1643
real 0m2.643s
user 0m1.304s
sys 0m1.306s



$ time grep -P 'post:content.*facebook' a_bunch_of_files* | wc -l
1643
real 0m29.542s
user 0m28.365s
sys 0m1.04s


Note that the -E flag uses “extended” regular expressions. All this does is change the default meaning of special characters. With the -E flag a pipe “|” means OR. Without the -E flag, a pipe “|” just represents the normal character, and to get the “special” meaning of OR, you have to escape it.