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

From: Stephane Chazelas <[email-masked]>
Date: Fri, 18 Nov 2016 16:37:54 +0000

2016-11-18 16:24:02 +0000, Stephane Chazelas:
[...]
> Why would it make it slower. AFAICT, PCRE_MULTILINE *adds*
> some overhead. It is really meant to be only about changing the
> behaviour of ^ and $.
[...]

Unsurprisingly,

where ~/a contains the output of find / -print0 (which is
typically what grep -z is used on) 304MiB, 3696401 records,
none of which contain a newline character in this instance.
with grep 2.10 (Ubuntu 12.04 amd64) in a UTF-8 locale:

$ time grep -Pz '(?-m)^/' ~/a > /dev/null
grep -Pz '(?-m)^/' ~/a > /dev/null  0.84s user 0.15s system 99% cpu 0.990 total
$ time grep -Pz '^/' ~/a > /dev/null
grep -Pz '^/' ~/a > /dev/null  0.87s user 0.12s system 99% cpu 0.989 total

Not much difference as "/" is found at the beginning of each
record.

$ time grep -Pz '(?-m)^x' ~/a > /dev/null
grep -Pz '(?-m)^x' ~/a > /dev/null  0.41s user 0.06s system 99% cpu 0.473 total
$ time grep -Pz '^x' ~/a > /dev/null
grep -Pz '^x' ~/a > /dev/null  0.81s user 0.04s system 99% cpu 0.854 total

PCRE_MULTILINE significantly slows things down as even though
"x" is not found at the beginning of the subject, grep still
needs to look for extra newline characters in the record.

-- 
Stephane