  We present a theoretical framework for the extraction and transformation of
text documents. We propose to use a two-phase process where the first phase
extracts span-tuples from a document, and the second phase maps the content of
the span-tuples into new documents. We base the extraction phase on the
framework of document spanners and the transformation phase on the theory of
polyregular functions, the class of regular string-to-string functions with
polynomial growth.
  For supporting practical extract-transform scenarios, we propose an extension
of document spanners described by regex formulas from span-tuples to so-called
multispan-tuples, where variables are mapped to sets of spans. We prove that
this extension, called regex multispanners, has the same desirable properties
as standard spanners described by regex formulas. In our framework, an
Extract-Transform (ET) program is given by a regex multispanner followed by a
polyregular function.
  In this paper, we study the expressibility and evaluation problem of ET
programs when the transformation function is linear, called linear ET programs.
We show that linear ET programs are equally expressive as non-deterministic
streaming string transducers under bag semantics. Moreover, we show that linear
ET programs are closed under composition. Finally, we present an enumeration
algorithm for evaluating every linear ET program over a document with linear
time preprocessing and constant delay.
