For using RegEx in Nintext, I'd recommend reading Vadim's post here. Regular Expressions in Nintex Workflow.  Then for figuring out specific patterns, I'd recommend using this site. RegExr: Learn, Build, & Test RegEx.  

please clarify what exactly you'd like to achieve with regex.

Maybe you can "split" your result to more Nintex Workflow actions. In the first action you can "get YEAR" as yyyy, in the second action you can "get that 00001" - if it is so, that the last number should contain 5 numbers, starting with zero(s). And in the last action you can concate all variables together.