Making the indentation of a multi-line paragraph equal to the indentation of the first line

Question

I have the following input that I receive from a third-party program:

START OF SECTION    
    This is a long sentence; really long that when rendered by PDFLatex it becomes a paragraph that spans a number of lines. Unfortunately, except the first line, all remaining lines will start from the very left of the document, but I want them to start from the indentation position of the first word of the paragraph.
    START OF SECTION        
        This is a long sentence; really long that when rendered by PDFLatex it becomes a paragraph that spans a number of lines. Unfortunately, except the first line, all remaining lines will start from the very left of the document, but I want them to start from the indentation position of the first word of the paragraph.
        START OF SECTION            
            This is a long sentence; really long that when rendered by PDFLatex it becomes a paragraph that spans a number of lines. Unfortunately, except the first line, all remaining lines will start from the very left of the document, but I want them to start from the indentation position of the first word of the paragraph.
        END OF SECTION
    END OF SECTION
END OF SECTION

I want to generate a PDF that looks exactly like is shown below: The indentation of each paragraph (the text inside the SECTION delimiters) follows the indentation of the first word/line of that paragraph.

I do not produce the text myself, like I said it is fed to me by another tool; so I cannot use enumeration unless I parse it and rewrite it with nested enumeration -- but I am trying to leave this as the very last resort.

Can anybody tell me the magic thing to do in order to instruct Latex to indent every paragraph according to its first line's indentation?

Many thanks.

Answer 1

\documentclass{article}

\begin{document}


\parindent0pt
\catcode`\ \active
\def\par{\endgraf\leftskip=0pt\relax}%
\obeylines
\def {\ifvmode\advance\leftskip5pt\relax\else\space\fi}%


START OF SECTION    
    This is a long sentence; really long that when rendered by PDFLatex it becomes a paragraph that spans a number of lines. Unfortunately, except the first line, all remaining lines will start from the very left of the document, but I want them to start from the indentation position of the first word of the paragraph.
    START OF SECTION        
        This is a long sentence; really long that when rendered by PDFLatex it becomes a paragraph that spans a number of lines. Unfortunately, except the first line, all remaining lines will start from the very left of the document, but I want them to start from the indentation position of the first word of the paragraph.
        START OF SECTION            
            This is a long sentence; really long that when rendered by PDFLatex it becomes a paragraph that spans a number of lines. Unfortunately, except the first line, all remaining lines will start from the very left of the document, but I want them to start from the indentation position of the first word of the paragraph.
        END OF SECTION
    END OF SECTION
END OF SECTION
\end{document}

It should work (I think) in RTL.

Vertical mode detects that you are between paragraphs, in that mode the space is not output at all, it is just used to increase the leftskip (oh, you may need rightskip) then once the paragraph starts and you are in horizontal mode a space expands to \space which is a normal space.

Then at the linebreak (end of paragraph here) \leftskip is reset to 0 ready for the next set of spaces to increase it to the value needed for the next paragraph.