Regex Syntax changes between POSIX and PCRE

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.

"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.