\zs not working in syntax file?

Question

Udated 2023-09-11: Changed example to asterisk as it is simpler and the question now refers to \zs instead of \ze as \zs seems to be the issue.

I am trying to adapt the VIM Markdown syntax file so that its behavior for italics with asterisks is more like the Github Markdown editor.

This pattern works exactly the way I want for matching italics:

syn match markdownItalic "\*\w\(\([^\n]\|\n[^\n]\)\{-}\)\w\*" contains=markdownItalicAsteriskStart,markdownItalicAsteriskEnd

To conceal the asterisks, I also need two patterns that exactly match the single asterisk characters that start and end the italics section. Trailing/leading whitespace NOT included. I have solved this, too. This is the search pattern I have found for finding the ending asterisk:

\(\*\w\(\([^\n]\|\n[^\n]\)\{-}\)\w\)\zs\*

Unfortunately this only works when I use it interactively in VIM, but not when I use it in markdown.vim (s:conceal is a variable set analogous to s:concealends in the original):

exe 'syn match markdownItalicAsteriskEnd "\(\*\w\(\([^\n]\|\n[^\n]\)\{-}\)\w\)\zs\*"' s:conceal

I suspect that it is \zs that does not work in my syntax file and have tried an alternative \@<= version. This almost works, but unfortunately cannot backtrack across more than one line (documented limitation) and therefore does not work for paragraphs in italics of more than two lines:

\(\*\w\(\([^\n]\|\n[^\n]\)\{-}\)\w\)\@<=\*

Does anybody know what can cause \zs to fail in a Markdown syntax file?

Udate 2023-09-12: Some Tests and a Workaround

markdownItalicAsteriskStart

I have found three ways to detect the starting asterisk. They all work perfectly.

Using \@=:

\*\(\w\(\([^\n]\|\n[^\n]\)\{-}\)\w\*\)\@=

Using \ze:

\*\ze\w\(\([^\n]\|\n[^\n]\)\{-}\)\w\*

With offset:

exe 'syn match markdownItalicAsteriskStart "\*\w\(\([^\n]\|\n[^\n]\)\{-}\)\w\*"ms=s,me=s+1' s:conceal

markdownItalicAsteriskEnd

Knowing three ways to solve the starting asterisk I thought I would try the corresponding methods for the terminating asterisk. Unfortunately only one works for me.

Using \@<= (works, but limited to two lines as stated in the documentation):

\(\*\w\(\([^\n]\|\n[^\n]\)\{-}\)\w\)\@<=\*

Using \zs (works perfectly in an interactive session, but does nothing when used in the syntax file; this is the subject of the question):

\*\w\(\([^\n]\|\n[^\n]\)\{-}\)\w\zs\*

With offset (does not work):

exe 'syn match markdownItalicAsteriskEnd "\*\w\(\([^\n]\|\n[^\n]\)\{-}\)\w\*"ms=e-1,me=e' s:conceal

The reason why this version with offsets will not work across newlines is documented here. But it does nothing at all for me and I have no idea why.

Workaround

This is the workaround I will use. It works very well with the only limitation that paragraphs in italics cannot span more than two lines. Quite OK for me.

let s:conceal = ''
let s:concealends = ''
if has('conceal') && get(g:, 'markdown_syntax_conceal', 1) == 1
  let s:conceal = ' conceal'
  let s:concealends = ' concealends'
endif
exe 'syn match markdownItalicAsteriskStart "\*\(\w\(\([^\n]\|\n[^\n]\)\{-}\)\w\*\)\@="' s:conceal
exe 'syn match markdownItalicAsteriskEnd "\(\*\w\(\([^\n]\|\n[^\n]\)\{-}\)\w\)\@<=\*"' s:conceal
syn match markdownItalic "\*\w\(\([^\n]\|\n[^\n]\)\{-}\)\w\*" contains=markdownItalicAsteriskStart,markdownItalicAsteriskEnd