PCRE before 8.38 mishandles the /(?|(\k'Pm')|(?'Pm'))/ pattern and related patterns with certain forward references, which allows remote attackers to cause a denial of service (buffer overflow) or possibly have unspecified other impact via a crafted regular expression, as demonstrated by a JavaScript RegExp object encountered by Konqueror.
https://security.gentoo.org/glsa/201607-02
https://bto.bluecoat.com/security-advisory/sa128
https://access.redhat.com/errata/RHSA-2016:1132
http://www.securityfocus.com/bid/85572
http://www.oracle.com/technetwork/topics/security/linuxbulletinapr2016-2952096.html
http://www.openwall.com/lists/oss-security/2015/11/29/1
http://www-01.ibm.com/support/docview.wss?uid=isg3T1023886
http://vcs.pcre.org/pcre/code/trunk/ChangeLog?view=markup