I have a content like below from that I want to select only the year information as string values.
Given this input string:
For example, one of the major problems is that the observer dynamics are assumed to be faster than that of the state feedback controller, which will cause numerical difficulty (Astrom and Wittenmark, 1997). What's more, when the process constraints are activated, the significant deterioration of closed-loop control performance will be clearly witnessed as kind of nonlinearity is dominating the control system (Tenny, Rawlings, and Wright, 2004). Therefore, it is necessary to develop methods capable of tackling the above problems.
I would like to extract 1997 and 2004.
Year information occurs only in four digits
Most probably it ends with full stop (period) or close bracket
It may occur at any place within the paragraph