Non-Capturing Group(?:\d+\s+days,)? ?repeats the group contents below at most once:
\d+a Unicode decimal digit, repeated at least once
\s+a Unicode whitespace character, repeated at least once
days,the literal text days, (case sensitive) \s*a Unicode whitespace character, repeated any number of times
\d{1,2}a Unicode decimal digit, repeated between 1 and 2 times
:the literal : (5810 / 728 / 3A16)
\d{1,2}a Unicode decimal digit, repeated between 1 and 2 times