---input---
f"a={d[a]}"

---tokens---
'f'           Literal.String.Affix
'"'           Literal.String.Double
'a='          Literal.String.Double
'{'           Literal.String.Interpol
'd'           Name
'['           Punctuation
'a'           Name
']'           Punctuation
'}'           Literal.String.Interpol
'"'           Literal.String.Double
'\n'          Text.Whitespace