Vim regression bug demonstration

test_file

bla
_S_	 bla

test_vimrc

set runtimepath=$VIM/vimfiles,$VIMRUNTIME,$VIM/vimfiles/after
set nocompatible
set listchars=tab:ab
set linebreak
set list
set conceallevel=2
let g:vimwiki_rxItalic = '\%(^\|\s\|[[:punct:]]\)\@<='.
      \'_'.
      \'\%([^_`[:space:]][^_`]*[^_`[:space:]]\|[^_`[:space:]]\)'.
      \'_'.
      \'\%([[:punct:]]\|\s\|$\)\@='
syn match VimwikiItalicChar contained /_/ conceal
execute 'syntax match VimwikiItalic /'.g:vimwiki_rxItalic.'/ contains=VimwikiItalicChar'