PCRE 7.8 and 8.32 through 8.37, and PCRE2 10.10 mishandle group empty matches, which might allow remote attackers to cause a denial of service (stack-based buffer overflow) via a crafted regular expression, as demonstrated by /^(?:(?(1)\.|([^\\W_])?)+)+$/.
rhn.redhat.com/errata/RHSA-2016-1025.html
rhn.redhat.com/errata/RHSA-2016-2750.html
vcs.pcre.org/pcre?view=revision&revision=1566
www-01.ibm.com/support/docview.wss?uid=isg3T1023886
www.openwall.com/lists/oss-security/2015/06/03/7
www.oracle.com/technetwork/topics/security/linuxbulletinapr2016-2952096.html
www.securityfocus.com/bid/75018
access.redhat.com/errata/RHSA-2016:1132
bugs.exim.org/show_bug.cgi?id=1638
bugzilla.redhat.com/show_bug.cgi?id=1228283