For example symbol * and +. Like net.Framework C#. This example should work sucessfully:
PCRE underpins the regular expression support in R, PHP, Delphi, MonetDB and other sofware projects - so it's pretty widely used.
This page suggests that the PCRE library supports variable-length look-behind, albeit it doesn't support arbitrary regexes within look-behinds.
PS: The (http://regular-expressions.info) website is an interesting source of information about various regular expression engines/libraries.