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.