If it can be parsed forwards it can be parsed backwards, although it might not be immediately possible to do so. The parser might have to check to see if it needs to re-parse a section, but I don't think there's any syntax we could introduce that is impossible to parse backwards. It's a difficulty thing.
However, a regex syntax would be much more difficult to parse backwards; I think this is a convincing enough argument that only simple raw strings should be implemented.
Pretty much every modern language that has regex has raw string literals. Heck, when regex was added to the C++ standard, raw string literals where added in the same spec.