Function try_parse_substitute_partial 

pub fn try_parse_substitute_partial(input: &str) -> Option<SubstitutePartial>

Best-effort parse of a partial :s / :%s command line. Used by the live-preview path: as the user types, we want to highlight matches of the in-progress pattern even before the second / or closing / is typed. Backslash-escapes are honored the same way try_parse_substitute honors them, so a partial \/ doesn’t flip the field state mid-stream.