It is unclear from the specification whether raw tab characters are allowed inside multi-line basic strings.

On the one hand, the section Multi-line basic strings states that "All other whitespace and newline characters remain intact.". This suggests that all whitespace is allowed inside the string. And whitespace has already been defined as tabs and spaces.

On the other hand, the same section states that "Any Unicode character may be used except those that must be escaped ... (U+0000 to U+001F, U+007F)".

So which is it ? Allowed because whitespace, or forbidden because control character ?

The ABNF definition forbids raw tab characters inside multi-line basic strings, but the ABNF is currently not authoritative.