parseDuration

Parses a short duration string like 24h24h, 30m30m, 90s90s, 7d7d, or 500ms500ms into milliseconds.

The grammar is one or more digits followed by one lowercase unit (msms, ss, mm, hh, dd). Anything else returns nullnull, including a bare number, an unknown unit, an uppercase unit, surrounding whitespace, a fractional value, and a result of zero. The return is a positive number or nullnull, never 00 or NaNNaN, so a malformed input can never read as a valid duration.