---input---
f"x = {x:+3}"

---tokens---
'f'           Literal.String.Affix
'"'           Literal.String.Double
'x = '        Literal.String.Double
'{'           Literal.String.Interpol
'x'           Name
':'           Literal.String.Interpol
'+3'          Literal.String.Double
'}'           Literal.String.Interpol
'"'           Literal.String.Double
'\n'          Text.Whitespace