Saw @arunsrinivasan's great vignette on reshaping in data.table and the internally implemented patterns function was mentioned; I spent a while searching around for how exactly it works before finally confirming it's accepting regex here.

?patterns and ??patterns turn up nothing; I guess it would be good to add a snippet about what exactly patterns is doing under ?melt (or under ?patterns if this syntax is acceptable to other data.table functions).