The pcre_compile2 function in PCRE before 8.37 allows context-dependent attackers to compile incorrect code and cause a denial of service (out-of-bounds read) via regular expression with a group containing both a forward referencing subroutine call and a recursive back reference, as demonstrated by "((?+1)(\1))/".
https://www.pcre.org/original/changelog.txt
https://fortiguard.com/zeroday/FG-VD-15-016
http://lists.opensuse.org/opensuse-updates/2015-05/msg00014.html