My feedback to your questions:

In addition, please recompile grep with `-g' and if it is reproduced,
get backtrace from a core file with gdb and send it to us.

  $ CFLAGS=-g ./configure OPTIONS
  $ make
  $ cat grepTxt | grep -P 'b(?:(?!tom).)+'
  $ gdb src/grep core.*

  (gdb) bt

Recompiling

# Core files are not dumped by default, enabling that.
€ ulimit -c unlimited
€ ulimit -a
core file size          (blocks, -c) unlimited
...

# Fresh compile
€ tar xf grep-2.21.tar       # new fresh directory.
€ cd grep-2.21
€ CFLAGS=-g ./configure --prefix=/tmp/segfaultGrep/grep/
...
€ make
...
€ cat ../../grepTxt | ./src/grep -qP 'b(?:(?!tom).)+'
../bin/grep: warning: GREP_OPTIONS is deprecated; please use an alias or script
Segmentation fault (core dumped)

# Only one core file is found, but not matching 'core.*'
# I have no experience with gdb, so I cannot really judge gdb output
€ find /tmp/segfaultGrep/ | grep -i core
/tmp/segfaultGrep/grep/grep-2.21/core
€ gdb src/grep core
GNU gdb (Ubuntu 7.7.1-0ubuntu5~14.04.2) 7.7.1
...
This GDB was configured as "x86_64-linux-gnu".
...
Reading symbols from src/grep...done.
[New LWP 1357]
Core was generated by `./src/grep -qP b(?:(?!tom).)+'.
Program terminated with signal SIGSEGV, Segmentation fault.
#0 0x00007fd8a278054a in ?? () from /lib/x86_64-linux-gnu/libpcre.so.3
(gdb) bt
#0 0x00007fd8a278054a in ?? () from /lib/x86_64-linux-gnu/libpcre.so.3
#1 0x00007fd8a2780ecb in ?? () from /lib/x86_64-linux-gnu/libpcre.so.3
#2 0x00007fd8a278dcfa in ?? () from /lib/x86_64-linux-gnu/libpcre.so.3
#3 0x00007fd8a278dcfa in ?? () from /lib/x86_64-linux-gnu/libpcre.so.3
#4 0x00007fd8a278dcfa in ?? () from /lib/x86_64-linux-gnu/libpcre.so.3
...
#16890 0x00007fd8a278dcfa in ?? () from /lib/x86_64-linux-gnu/libpcre.so.3
#16891 0x00007fd8a278dcfa in ?? () from /lib/x86_64-linux-gnu/libpcre.so.3
#16892 0x00007fd8a278dcfa in ?? () from /lib/x86_64-linux-gnu/libpcre.so.3
#16893 0x00007fd8a2791221 in pcre_exec () from /lib/x86_64-linux-gnu/libpcre.so.3
#16894 0x000000000041621d in Pexecute (
    buf=0x13fc000 "b3VuZDwvaDE+Iik7fQoKJGxhbmd1YWdlPSdlbmcnOwoKJGF1dGggPSAwOwoKJG5hbWU9Jyc7IAokcGFzcz0nJzsKCi8vcnVfUlUsIC8vcnVfUlUuY3AxMjUxLCAvL3J1X1JVLmlzbzg4NTk1LCAvL3J1X1JVLmtvaThyLCAvL3J1X1JVLnV0ZjgKQHNldGxvY2FsZShM"..., size=16893, match_size=0x7fffa0333150, start_ptr=0x0) at pcresearch.c:215
#16895 0x0000000000404b95 in grepbuf (
    beg=0x13fc000 "b3VuZDwvaDE+Iik7fQoKJGxhbmd1YWdlPSdlbmcnOwoKJGF1dGggPSAwOwoKJG5hbWU9Jyc7IAokcGFzcz0nJzsKCi8vcnVfUlUsIC8vcnVfUlUuY3AxMjUxLCAvL3J1X1JVLmlzbzg4NTk1LCAvL3J1X1JVLmtvaThyLCAvL3J1X1JVLnV0ZjgKQHNldGxvY2FsZShM"..., lim=0x14001fd "\377") at grep.c:1232
#16896 0x0000000000404fcb in grep (fd=0, st=0x7fffa0333260) at grep.c:1350
#16897 0x0000000000405aad in grepdesc (desc=0, command_line=true) at grep.c:1639
#16898 0x0000000000405ca4 in grep_command_line_arg (arg=0x42440e "-") at grep.c:1686
#16899 0x000000000040774d in main (argc=4, argv=0x13fa080) at grep.c:2564

# OK, I guess I know enough about gdb to understand we're missing symbols in the output ...
€ sudo apt-get install libpcre3-dbg
...

# Working around almost 10 megabyte output text...
€ echo bt > bt
€ gdb src/grep core --batch -x bt > /tmp/gdb.out
484     pcre_exec.c: No such file or directory.
# http://wirespeed.xs4all.nl/media/segfaultGrep/

I checked on a VM on my laptop, but I can't reproduce the issue there. It is 32 bit Ubuntu 14.04.1

I double checked the intermittent behavior on my PC, don't have a reason to believe it is hardware related because my system is otherwise stable. I found these messages in 'dmesg' after checking the command couple times, notice the 'libpcre'.

€ dmesg

[2119696.762373] grep[6488]: segfault at 7fff51de7fe8 ip 00007f5db8a3454a sp 00007fff51de7fe0 error 6 in libpcre.so.3.13.1[7f5db8a21000+3d000]
[2119697.880722] grep[6569]: segfault at 7ffffcd28ff8 ip 00007f00ae1ea53d sp 00007ffffcd28e50 error 6 in libpcre.so.3.13.1[7f00ae1d7000+3d000]
[2119698.811133] grep[6646]: segfault at 7fff6252fec8 ip 00007f0c7545754a sp 00007fff6252fec0 error 6 in libpcre.so.3.13.1[7f0c75444000+3d000]
[2119699.674442] grep[6708]: segfault at 7fff5fdcdee8 ip 00007fb27fa3b54a sp 00007fff5fdcdee0 error 6 in libpcre.so.3.13.1[7fb27fa28000+3d000]
[2119700.488632] grep[6770]: segfault at 7fff8fda7ff8 ip 00007f7791bc5ec6 sp 00007fff8fda8000 error 6 in libpcre.so.3.13.1[7f7791bb2000+3d000]
[2119701.274100] grep[6822]: segfault at 7fff29bd8f18 ip 00007fb0bd44d54a sp 00007fff29bd8f10 error 6 in libpcre.so.3.13.1[7fb0bd43a000+3d000]
[2119702.090632] grep[6890]: segfault at 7fffa33f8ee8 ip 00007fd4b786954a sp 00007fffa33f8ee0 error 6 in libpcre.so.3.13.1[7fd4b7856000+3d000]
[2119702.875331] grep[6943]: segfault at 7fffb0b0efd8 ip 00007fc2a9d7554a sp 00007fffb0b0efd0 error 6 in libpcre.so.3.13.1[7fc2a9d62000+3d000]
[2119715.819686] grep[7795]: segfault at 7fff20e78e68 ip 00007fd5162c854a sp 00007fff20e78e60 error 6 in libpcre.so.3.13.1[7fd5162b5000+3d000]
[2119717.595114] grep[7913]: segfault at 7fff2bc21fd8 ip 00007f30455ec54a sp 00007fff2bc21fd0 error 6 in libpcre.so.3.13.1[7f30455d9000+3d000]

I believe I mentioned libpcre3:amd64 1:8.31-2ubuntu2 before, but here it is for completeness.

€ locale

LANG=en_US.UTF-8
LANGUAGE=en
LC_CTYPE="en_US.UTF-8"
LC_NUMERIC=en_US.UTF-8
LC_TIME=en_US.UTF-8
LC_COLLATE="en_US.UTF-8"
LC_MONETARY=en_US.UTF-8
LC_MESSAGES="en_US.UTF-8"
LC_PAPER=en_US.UTF-8
LC_NAME=en_US.UTF-8
LC_ADDRESS=en_US.UTF-8
LC_TELEPHONE=en_US.UTF-8
LC_MEASUREMENT=en_US.UTF-8
LC_IDENTIFICATION=en_US.UTF-8
LC_ALL=

On 2014-12-20 21:57, Paul Eggert wrote:

I can't reproduce the problem on Ubuntu 14.10 (x86-64) or Fedora 20 (x86-64), with either the bundled grep or with GNU grep 2.21.

What locale are you running in? What's the output of the 'locale' command?

It could be a bug in grep, or in libpcre, or somewhere else. Ubuntu 14.10 is running libpcre3 1:8.35-3ubuntu1. I vaguely recall that they're doing something funky to prevent grep from depending on libpcre while still supporting -P, but I don't recall the details.