Re: string.gsub pattern question

[Duncan Cross <duncan.cross@...>]

On Tue, Sep 27, 2011 at 10:53 AM, Daniel Hertrich
<dhertrich@googlemail.com> wrote:
> Hi list,
>
> probably a trivial question to some of you, but I am stuck on this...
> I'd like to replace the endings of lines from a HTML-formatted string,
> using string.gsub(), according to the following criterion:
>
> Each line ending on "\n" only, but not on "<br>\n", shall be modified
> to end on "<br>\n"
> Or in words: All lines should end on "<br>\n", but some lack the
> "<br>", and if the "<br>" does not exist yet, generate it.
>
> Anyone can assist me with the pattern to use please?
>
> strung.gsub(strInput, ????, "<br>\n")
>
> Thank you!
> Daniel
>
>

I don't believe this can be done with a simple pattern replacement.
You have to move into using a custom callback function to do the
negative look-behind, something like:

string.gsub(strInput, "()\n", function(pos)
  if (pos < 5) or (string.sub(strInput, pos-4, pos-1) ~= '<br>') then
    return '<br>\n'
  end
  return '\n'
end)

-Duncan