  Regular expressions with capture variables, also known as regex-formulas,
extract relations of spans (intervals identified by their start and end
indices) from text. In turn, the class of regular document spanners is the
closure of the regex formulas under the Relational Algebra. We investigate the
computational complexity of querying text by aggregate functions, such as sum,
average, and quantile, on top of regular document spanners. To this end, we
formally define aggregate functions over regular document spanners and analyze
the computational complexity of exact and approximate computation. More
precisely, we show that in a restricted case, all studied aggregate functions
can be computed in polynomial time. In general, however, even though exact
computation is intractable, some aggregates can still be approximated with
fully polynomial-time randomized approximation schemes (FPRAS).
