Syntax: Stricter italic match: match '_that_' not 'a_ that _a'

See: https://github.github.com/gfm/#emphasis-and-strong-emphasis

`A single * character can open emphasis iff (if and only if) it is part of a left-flanking delimiter run.`

Tim Pope default is too permissive (leads to too much emphasis openener): `\S\@<=_\|_\S\@=` -> `_\S\@=`

autoload/vimwiki/u.vim

@@ -273,14 +273,11 @@ endfunction
 " Helper: Expand regex from reduced typeface delimiters
 " :param: list<list,delimiters>> with reduced regex
 " Return: list with extended regex delimiters (not inside a word)
-"   -- [['\*_', '_\*']] -> [['\S\@<=\*_\|\*_\S\@=', '\S\@<=_\*\|_\*\S\@=']]
+"   -- [['\*_', '_\*']] -> [['\*_\S\@=', '\S\@<=_\*']]
 function! vimwiki#u#hi_expand_regex(lst) abort
   let res = []
-  function! s:expand_regex(rx) abort
-    return '\S\@<=' .a:rx . '\|' . a:rx . '\S\@='
-  endfunction
   for delimiters in a:lst
-    call add(res, [s:expand_regex(delimiters[0]), s:expand_regex(delimiters[1])])
+    call add(res, [delimiters[0] . '\S\@=', '\S\@<=' . delimiters[1]])
   endfor
   return res
 endfunction

syntax/vimwiki_default.vim

@@ -16,23 +16,19 @@ let b:vimwiki_syntax_conceal = exists('+conceallevel') ? ' conceal' : ''
 let b:vimwiki_syntax_concealends = has('conceal') ? ' concealends' : ''
 
 " text: *strong*
-let s:default_syntax.dTypeface.bold = [
-      \ ['\S\@<=\*\|\*\S\@=', '\S\@<=\*\|\*\S\@='],
-      \ ]
+let s:default_syntax.dTypeface.bold = vimwiki#u#hi_expand_regex([
+      \ ['\*', '\*']])
 
 " text: _italic_
-let s:default_syntax.dTypeface.italic = [
-      \ ['\S\@<=_\|_\S\@=', '\S\@<=_\|_\S\@='],
-      \ ]
+let s:default_syntax.dTypeface.italic = vimwiki#u#hi_expand_regex([
+      \ ['_', '_']])
 
 " text: no underline defined
 let s:default_syntax.dTypeface.underline = []
 
 " text: *_bold italic_* or _*italic bold*_
-let s:default_syntax.dTypeface.bold_italic = [
-      \ ['\S\@<=\*_\|\*_\S\@=', '\S\@<=_\*\|_\*\S\@='],
-      \ ['\S\@<=_\*\|_\*\S\@=', '\S\@<=\*_\|\*_\S\@='],
-      \ ]
+let s:default_syntax.dTypeface.bold_italic = vimwiki#u#hi_expand_regex([
+      \ ['\*_', '_\*'], ['_\*', '\*_']])
 
 " generic headers
 let s:default_syntax.rxH = '='

test/syntax.vader

@@ -4,6 +4,24 @@
 # 1 Typeface {{{1
 #################
 
+# Emphasis stricker {{{2
+# See: https://github.github.com/gfm/#emphasis-and-strong-emphasis
+
+Given vimwiki (Emphasis and not):
+  this __bold__ ok
+  this _italic_ ok
+  t__ no bold __ t
+  t_ no ital _ t
+
+Execute (Set syntax markdown):
+  call SetSyntax('markdown')
+
+Execute (Assert Syntax extended types x 1):
+  AssertEqual 'VimwikiBold'        , SyntaxAt(1, 9)
+  AssertEqual 'VimwikiItalic'      , SyntaxAt(2, 9)
+  AssertEqual ''                   , SyntaxAt(3, 9)
+  AssertEqual ''                   , SyntaxAt(4, 9)
+
 # With vimwiki_hl_cb_checked {{{2
 
 Given vimwiki (task list with code):
@@ -102,17 +120,17 @@ Given vimwiki (Basic Types nested in extended):
   From ,,sub to _italic with en_  aaaaaaa,,      morF
   From $eq to  **boldboldboldbo** aaaaaaaaa         $
   From ^super t *italic aaaaaaa*aaaaaaaaaaaaaaaaaaaaa
-  From ~~strike__ bbbbbbbbbbbbb__ssssssssssssssssss~~
+  From ~~strike__bbbbbbbbbbbbb__ssssssssssssssssss~~
 
 Execute (Set syntax markdown):
   call SetSyntax('markdown')
 
-Execute (Assert Syntax basic  types nested in extended):
-  AssertEqual 'VimwikiBold'        , SyntaxAt(1, 23)
-  AssertEqual 'VimwikiItalic'      , SyntaxAt(2, 23)
-  AssertEqual 'textSnipTEX'        , SyntaxAt(3, 23)
-  AssertEqual 'VimwikiItalic'      , SyntaxAt(4, 23)
-  AssertEqual 'VimwikiBold'        , SyntaxAt(5, 23)
+Execute (Assert Syntax basic types nested in extended):
+  AssertEqual 'VimwikiBold1'        , SyntaxAt(1, 23) . 1
+  AssertEqual 'VimwikiItalic2'      , SyntaxAt(2, 23) . 2
+  AssertEqual 'textSnipTEX3'        , SyntaxAt(3, 23) . 3
+  AssertEqual 'VimwikiItalic4'      , SyntaxAt(4, 23) . 4
+  AssertEqual 'VimwikiBold5'        , SyntaxAt(5, 23) . 5
 
 Given vimwiki (Try to nest in code):
   From `codeto__no onenest in code__ end`