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

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