Im using this string in javascript to validate a date field upon blur event. Im no regex expert but what am I missing that is keeping this from working? I need it to validate a mm/dd/yyyy, m/d/yyyy, mm/d/yyyy, m/dd/yyyy and also to include leap year.

This is the string I have so far but it lets 13/10,2020 pass.

Code:
((0?|0[13578]|1[02])[\/.](0[1-9]|[12][0-9]|3[01])[\/.](19|20)[0-9]{2})
|
((0?|0[469]|11)[\/.](0[1-9]|[12][0-9]|30)[\/.](19|20)[0-9]{2})|((02)[\/.](0[1-9]|1[0-9]|2[0-8])[\/.](19|20)[0-9]{2})
|
((2)|(02)[\/.]29[\/.](((19|20)(04|08|[2468][048]|[13579][26]))|2000))

Think this should work for just the month validation (13578 months only to break this down simplier)?
(([13578])|0[13578]|1[02])

Thanks