Stephane Chazelas wrote:
2016-11-18 08:48:04 -0800, 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 might have been the case a long time ago, as I remember
some discussion about it as it explained some wrong information
in the documentation, but as far as I and gdb can tell, grep
2.26 at least call pcre_exec for every line of the input with
grep -P.
Although that was true starting with commit
a14685c2833f7c28a427fecfaf146e0a861d94ba (2010-03-04), it became false starting
with commit 9fa500407137f49f6edc3c6b4ee6c7096f0190c5 (2014-09-16).
If it didn't
echo test | grep -P '\n$'
would match.
No, because grep omits the trailing newline in that particular input. And for
this example:
printf 'test\n\n' | grep -p '\n$'
grep passes "test\n" to jit_exec, determines that jit_exec returns a match that
crosses a line boundary, and rejects the match.
