Function parse_relaxed 

pub fn parse_relaxed(input: &str) -> ParseResult<Vec<Token>, EmptyErr>

Parse a format string with relaxed idents. Only the initial identifier of a variable is relaxed,

A relaxed identifier doesn’t start with a numeric character and contains any characters that are

• not control chars
• not a whitespace
• not a colon (:)
• no braces ({ or })