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

"Regular Expressions as used in R"

I didn't find a comparison chart but I did find Regular Expressions as used in R which has a lot of information near the top part of the document related to POSIX and the bottom half details pcre and also gives links to the standards.

Answer 2

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.