nl_tokens: https : // html . spec . whatwg . org / multipage / common - microsyntaxes . html#parse - a - week - string
pl_tokens: function parseWeekString ( str ) { const matches = weekRe . exec ( str ) ; if ( ! matches ) { return null ; } const <mask> = Number ( matches [ 1 ] ) ; if ( year <= 0 ) { return null ; } const week = Number ( matches [ 2 ] ) ; if ( week < 1 || week > weekNumberOfLastDay ( year ) ) { return null ; } return { year , week } ; }