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_])?)+)+$/. http://web.nvd.nist.gov/view/vuln/detail?vulnId=CVE-2015-3217
