See http://en.cppreference.com/w/cpp/language/string_literalCloses#877.
---
This contains a pretty ugly hack to fetch the previous character, in
order not to get fooled by string concatenation hidden behind a macro,
like in `FOUR"five"`, which is not a raw string literal but simply the
identifier `FOUR` followed by the string `"five"`.
While this may sound uncommon, it is not and lead to complaints [2][3]
when Scintilla [1] broke this when they introduced C++11 raw string
literal support themselves.
The implementation here still contains a bug with line continuations: a
raw literal of the form:
```c
const char *str = R\
"xxx(...)xxx";
```
is not properly recognized as such, although it's perfectly valid (yet
probably very uncommon). For the record, Scintilla has also suffers
from this but nobody complained about it yet.
[1] http://scintilla.org/
[2] https://sourceforge.net/p/scintilla/bugs/1207/
[3] https://sourceforge.net/p/scintilla/bugs/1454/
a14aa908c5