[Tex/LaTex] Are LaTeX3 regular expressions able to match characters at the start of a line

Question

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I would like to use LaTeX3 regular expressions to extract the "lines" from text like the following:

- First line
- second line ending with \LaTeX
- third line with - in the middle
- fourth line that
goes on for a bit

By a "line" I mean text that starts with a dash - that occurs at the beginning of a "physical line", and then continues until the next such dash, or the end of the string.

My first guess was that something like the following would work:

\regex_split:nVN {  [\f\n\r]\s+-\s+  } { \Lines } \l_tmp_seq

My understanding is that this should match a "newline character" followed by - that is surrounded by white-space on both sides. Unfortunately, this fails horribly, producing only one match:

As this didn't work I thought that I would try a very simple-minded approach and just split the string on white-space surrounded dashes -. This would give the wrong result on lines like the third line above, but I can use -- in the middle of the line so this is a workable alternative. As \s matches white space, the following should split lines on -:

\regex_split:nVN { \s+-\s+  } { \Lines } \l_tmp_seq

This is better, but the results are still not perfect because, in addition to the expected problem on line 3, having \LateX at the end of a line confuses \regex_split presumably because macros tends to swallow spaces:

The best solution that I have found is using:

\regex_split:nVN { \B-\B } { \Lines } \l_tmp_seq

This produces the output that I was expecting from the last regex:

That is, we are getting an unwanted match for the - in the middle of the third line but this is otherwise OK.

Question Is there a regular expression that does not return any false matches? Initial and/or final matching empty sequences are allowed.

[Aside: I am not worried about the "empty" initial sequences above because I know that can get rid of them using

\seq_pop_left:NN \l_tmp_seq \l_tmpa_tl % pop initial blank

Terminating empty sequences can, of course, be dealt with in almost exactly the same way using \seq_pop_right:NN.]

Here is a full MWE to produce the examples above:

\documentclass{article}
\usepackage{xparse}
\parindent=0pt

\newcommand\BS{$\backslash$}
\newcommand\Test[1]{\textbf{Testing:}\quad\texttt{#1}}
\begin{document}

\newcommand\Lines{
    - First line
    - second line ending with \LaTeX
    - third line with -- in the middle
    - fourth line that
    goes on for a bit
}

\ExplSyntaxOn
\seq_new:N \l_tmp_seq
\cs_generate_variant:Nn \regex_split:nnN { nVN }

\Test{[\BS f\BS n\BS r]\BS s+-\BS s+}
\regex_split:nVN { [\f\n\r]\s+-\s+  } { \Lines } \l_tmp_seq
\begin{itemize}\item\seq_use:Nn \l_tmp_seq {\item }\end{itemize}

\Test{\BS s-\BS s}
\regex_split:nVN { \s-\s  } { \Lines } \l_tmp_seq
\begin{itemize}\item\seq_use:Nn \l_tmp_seq {\item }\end{itemize}

\Test{\BS B-\BS B}
\regex_split:nVN { \B-\B  } { \Lines } \l_tmp_seq
\begin{itemize}\item\seq_use:Nn \l_tmp_seq {\item }\end{itemize}

\ExplSyntaxOff

\end{document}

Escaping the - in the regular expressions doesn't help either.

Finally, the problem may be that the newline characters in \Lines are stripped when it is defined. As far as I can see this is OK because the code above gives similar output. In particular, if I use:

\documentclass{article}
\usepackage{xparse}
\parindent=0pt

\begingroup\obeylines
\gdef\Lines{
    - First line
    - second line ending with \LaTeX
    - third line with -- in the middle
    - third line that
    goes on for a bit
}
\endgroup

\begin{document}

  \ExplSyntaxOn
  \seq_new:N \l_tmp_seq
  \cs_generate_variant:Nn \regex_split:nnN { nVN }
  \regex_split:nVN { [\f\n\r]\s+\-\s+  } { \Lines } \l_tmp_seq

  Sequence~length:~ \seq_count:N \l_tmp_seq.
  \begin{itemize}\item\seq_use:Nn \l_tmp_seq {\item }\end{itemize}
  \ExplSyntaxOff

\end{document}

then I get the output:

In practice, I am getting \Lines as the \BODY of a environ environment, so I don't really have the luxury of this definition…

Answer 1

You need obeylines or something equivalent so that the newline char doesn't disappear. The following splits the lines (nearly) as wanted. Instead of \^^M one can also use \r (I only used the first because \show\Lines showed ^^M).

This creates this sequence:

The sequence \l_tmp_seq contains the items (without outer braces):
>  {First line}
>  {second line ending with \LaTeX }
>  {third line with -- in the middle}
>  {fourth line that^^Mgoes on for a bit^^M}.
<recently read> }

One problem could be that the fourth item contains a ^^M and so a probably unwanted line break.

\documentclass{article}
\usepackage{xparse}
\parindent=0pt

\newcommand\BS{$\backslash$}
\newcommand\Test[1]{\textbf{Testing:}\quad\texttt{#1}}
\begin{document}

\begingroup
\obeylines
\gdef\Lines{
    - First line
    - second line ending with \LaTeX
    - third line with -- in the middle
    - fourth line that
    goes on for a bit
}

\endgroup

\ExplSyntaxOn
\seq_new:N \l_tmp_seq
\cs_generate_variant:Nn \regex_split:nnN { nVN }

\Test{}\verb+^^M\-+
\regex_replace_once:nnN { \A\^^M?\-\s* } {} \Lines %to get rid of the first ^^M
\regex_split:nVN { \^^M\-\s* } \Lines \l_tmp_seq

\begin{itemize}\item\seq_use:Nn \l_tmp_seq {\item }\end{itemize}


\ExplSyntaxOff

\end{document}