regexp capturing issue

Question

The following regexp code gives the result

\documentclass[11pt]{book} % use larger type; default would be 10pt


\usepackage{pgffor}
\usepackage{l3regex,xparse}
\usepackage{etextools}
\begin{document}  

\ExplSyntaxOn
\seq_new:N \l_uiy_result_seq
\NewDocumentCommand {\UiySplit } { m }
  {
    %\regex_extract_all:nnN { \D+ | \d+(?:\.\d*)? } {#1} \l_uiy_result_seq
    \regex_extract_all:nnN {(f)(\d+(?:\.\d*)?)(s)(\d+(?:\.\d*)?)} {#1} \l_uiy_result_seq
    \seq_map_inline:Nn \l_uiy_result_seq { item:~##1\par }
  }
\ExplSyntaxOff

\UiySplit{f234s222}

\end{document}

gives the following output

item: f234s222
item: f
item: 234
item: s
item: 222

Why is it capturing the whole string and outputting it?

Similarly:

\regex_extract_all:nnN {(f|s)(\d+(?:\.\d*)?){1,2}} {#1} \l_uiy_result_seq

is outputing

item: f234
item: f
item: 234
item: s222
item: s
item: 222

the first and fourth shouldn't be there? (well, I don't want them captured)

Answer 1

I have the feeling that your task would be best done using a grammar. Unfortunately, there is no such package yet out there (expect one some time next year), so we are stuck with regular expressions only. There may be hope using LPeg if you use LuaTeX, but I know nothing about those developments.

First, why the behavior: I simply followed Perl. The main task of the extract functions is to extract what they match, in your case, the whole string, and a secondary effect is that capturing groups are also, well, captured. But as far as I know, Perl does not provide a way to not capture the whole match, only to avoid capturing individual groups. If I am wrong and there is an accepted syntax for that task, I will most definitely consider adding it to l3regex.

In your situation (and in fact, in most), the various groups that you are capturing have different statuses [hmm... makes me think that perhaps returning a sequence of sequences may make more sense]. So when looping through the sequence, you would anyways have to keep track of the index, checking what its value modulo 4 (or whatever) is, or to remove items from the beginning of the sequence one by one. The fact that the whole match is captured simply means replacing 4 by 5, or discarding one more item from the sequence.

You may be interested in doing a replacement instead of an extraction, for an example applicable here see the code in this other answer of mine.

Answer 2

The regex maps the result into a sequence.

  \seq_map_inline:Nn \l_uiy_result_seq { item:~##1\par }

You can extract the values of interest from the sequence, by referring to the LaTeX3 manual for sequences.