On this reference: https://en.cppreference.com/w/cpp/language/string_literal, raw string is defined as:
prefix(optional) R"d-char-sequence(optional)(r-char-sequence(optional))d-char-sequence(optional)"
Example:
const char* s1 = R"foo(
Hello
World
)foo";
What is the purpose of the d-char-sequence ("foo" in the example above)?
The optional d-char-sequence is used to define the end marker of the raw string.
For example, if the raw string contains the substring )", then the line:
const char* s = R"(string with )" inside)";
will raise a syntax error. This can be fixed by using the optional d-char-sequence that is not met inside the raw string:
const char* s = R"uniq(string with )" inside)uniq";
It lets you embed )" in the literal, which would otherwise be considered the end of the literal.