---input---
fr"{header}:\s+"

---tokens---
'fr'          Literal.String.Affix
'"'           Literal.String.Double
'{'           Literal.String.Interpol
'header'      Name
'}'           Literal.String.Interpol
':'           Literal.String.Double
'\\'          Literal.String.Double
's+'          Literal.String.Double
'"'           Literal.String.Double
'\n'          Text.Whitespace