Regex Syntax changes between POSIX and PCRE

Question

We are currently in the process of upgrading our Varnish Cache servers. As part of the process, we upgraded only one of them to see how it behaves compared to the older vers

Answer 1

See Regular Expression Engine Comparison Chart maintained by Roger Qui which a copy of the information available in the original answer. (Credit to Uberhumus for the new link.)

[Original Answer] See Flavor Comparison at Regular-Expressions.info.