bug#22655: grep -Pz '^' now fails!

[Paul Eggert]

Stephane Chazelas wrote:
> Why would it make it slower. AFAICT, PCRE_MULTILINE *adds*
> some overhead.

As I understand it, PCRE_MULTILINE lets 'grep' apply a pattern to an entire 
buffer that contains many lines, and this lets PCRE efficiently find the first 
match in the whole buffer. If grep doesn't use PCRE_MULTILINE, grep would have 
to apply the pattern to each line separately, which could be significantly slower.

That being said, PCRE matching is pretty slow already, so perhaps we shouldn't 
worry about efficiency here.