Function format_pattern 

pub fn format_pattern(
    pattern: &str,
    f: &DateFields,
    names: &DateNames,
) -> String

A TR35 pattern over fields: y yy yyyy M MM MMM MMMM d dd E EEE EEEE h hh H HH m mm s ss a, 'quoted' literals ('' = an apostrophe); any other letter run is copied as written. Digits are ASCII.