FzfBtags cannot list some tags in some case

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- [ ] Question
- [x] Bug
- [ ] Suggestion
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here is a C++ source file

```
#include

static std::string jstr = R"(
{
"name": "test",
"age": 18,
"params": "{
\"hobby\":\"basketball\"
}"
})";

int main()
{
std::cout << jstr << std::endl;
return 0;
}
```

`FzfBtags` shows `No tags found`. However, when I remove `\` in this line

```
\"hobby\":\"basketball\"
```
then, `FzfBtags` show correctly.

So, looks `\` in C++ `R"()"` makes `FzfBtags` not work