Updating version :
I've found an other expression which segfaults also PHP 5 :
preg_match("/^((?<!a).)*/",str_repeat('b',21236),$z);

preg_match("/(((?<!aaa).)*)(?<!aaa)aaa/",str_repeat(' ',10882).'aaa',$z);

crashes PHP4.3.9RC2

But not on php-4.3.2-11.1.ent (WBEL 3.0), the length
to trigger segmentation fault is about 19230.

The most funny thing is that the more closer to the limit, the more likely you will get a random segmentation fault.

Not only the above pattern will cause the error,
preg_match("/^( )*$/",str_repeat(' ',19250));
will too.

your last regex crashes PHP 5 also.

The segfault isn't in PHP but in pcre (this is quite normal due to the NFA nature of pcre).

Can't reproduce with any of dev versions (tried latest 4.3.10-dev, 5.1.0-dev & 5.0.3-dev under Linux). Please, try latest snapshots and tell me what version of pcre you're using (mine is 3.9) if you're still able to reproduce it.

Still segfault with PHP 4.3.10RC2 and PCRE Library Version 	4.5 01-December-2003

This is the standard "PCRE uses on-stack recursion" bug which has been filed and closed umpteen times.  To reproduce just increase the length of the string until exhausts your stack space.

One way PHP could mitigate the issue is to to set the match_limit field in the pcre_extra structure which puts a limit on the depth of the stack recursion.  

Still crashes with 4.4.1, increasing the length of the string.

Andrei says this is not possible to fix since we can't tell what the limit for recursion should be.

> It's not really useful since it doesn't exist. Try again..
You should have been a bit faster to look at it, while it was online.

I re-paste it here :

#0  0x080ad99c in match (eptr=0x8358fe4 ' ' <repeats 200 times>..., ecode=0x834faf5 "I", offset_top=6, md=0xbffaa264, ims=0, 
    eptrb=0x0, flags=2) at /home/xanthor/temp/web/php-4.4.1/ext/pcre/pcrelib/pcre_exec.c:357
357     {
(gdb) bt
#0  0x080ad99c in match (eptr=0x8358fe4 ' ' <repeats 200 times>..., ecode=0x834faf5 "I", offset_top=6, md=0xbffaa264, ims=0, 
    eptrb=0x0, flags=2) at /home/xanthor/temp/web/php-4.4.1/ext/pcre/pcrelib/pcre_exec.c:357
#1  0x080adb9b in match (eptr=Variable "eptr" is not available.
) at /home/xanthor/temp/web/php-4.4.1/ext/pcre/pcrelib/pcre_exec.c:747
#2  0x080b1222 in match (eptr=Variable "eptr" is not available.
) at /home/xanthor/temp/web/php-4.4.1/ext/pcre/pcrelib/pcre_exec.c:589
#3  0x080b2327 in match (eptr=Variable "eptr" is not available.
) at /home/xanthor/temp/web/php-4.4.1/ext/pcre/pcrelib/pcre_exec.c:1123
#4  0x080b1222 in match (eptr=Variable "eptr" is not available.
) at /home/xanthor/temp/web/php-4.4.1/ext/pcre/pcrelib/pcre_exec.c:589
#5  0x080b2327 in match (eptr=Variable "eptr" is not available.
) at /home/xanthor/temp/web/php-4.4.1/ext/pcre/pcrelib/pcre_exec.c:1123
#6  0x080b1222 in match (eptr=Variable "eptr" is not available.
) at /home/xanthor/temp/web/php-4.4.1/ext/pcre/pcrelib/pcre_exec.c:589
#7  0x080b2327 in match (eptr=Variable "eptr" is not available.
) at /home/xanthor/temp/web/php-4.4.1/ext/pcre/pcrelib/pcre_exec.c:1123
#8  0x080b1222 in match (eptr=Variable "eptr" is not available.
) at /home/xanthor/temp/web/php-4.4.1/ext/pcre/pcrelib/pcre_exec.c:589
#9  0x080b2327 in match (eptr=Variable "eptr" is not available.
) at /home/xanthor/temp/web/php-4.4.1/ext/pcre/pcrelib/pcre_exec.c:1123
#10 0x080b1222 in match (eptr=Variable "eptr" is not available.
) at /home/xanthor/temp/web/php-4.4.1/ext/pcre/pcrelib/pcre_exec.c:589
#11 0x080b2327 in match (eptr=Variable "eptr" is not available.
) at /home/xanthor/temp/web/php-4.4.1/ext/pcre/pcrelib/pcre_exec.c:1123
#12 0x080b1222 in match (eptr=Variable "eptr" is not available.
) at /home/xanthor/temp/web/php-4.4.1/ext/pcre/pcrelib/pcre_exec.c:589
#13 0x080b2327 in match (eptr=Variable "eptr" is not available.
) at /home/xanthor/temp/web/php-4.4.1/ext/pcre/pcrelib/pcre_exec.c:1123

And same thing again and again...

Andrei says this is not possible to fix since we can't tell what the limit for recursion should be.