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

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