RE2 regular expressions for Varnish

 Varnish Module (VMOD) for access to the Google RE2 regular expression
 engine. Varnish VCL uses the PCRE library (Perl Compatible Regular Expressions)
 for its native regular expressions, which runs very efficiently for many
 common uses of pattern matching in VCL, as attested by years of
 successful use of PCRE with Varnish.
 .
 But for certain kinds of patterns, the worst-case running time of the
 PCRE matcher is exponential in the length of the string to be matched.
 The matcher uses backtracking, implemented with recursive calls to the
 internal match() function. In principle there is no upper bound to the
 possible depth of backtracking and recursion, except as imposed by the
 varnishd runtime parameters pcre_match_limit and
 pcre_match_limit_recursion; matches fail if either of these limits are
 met. Stack overflow caused by deep backtracking has occasionally been
 the subject of varnishd issues.
 .
 RE2 differs from PCRE in that it limits the syntax of patterns so that
 they always specify a regular language in the formally strict sense.
 Most notably, backreferences within a pattern are not permitted, for
 example (foo|bar)\1 to match foofoo and barbar, but not foobar
 or barfoo. See the link in SEE ALSO for the specification of RE2
 syntax.
 .
 This means that an RE2 matcher runs as a finite automaton, which
 guarantees linear running time in the length of the matched string.
 There is no backtracking, and hence no risk of deep recursion or stack
 overflow.