Merge 9869533e28 into f9f845f28f

Shortcut reference links are defined in three of the most common
Markdown implementations:

- [Pandoc](https://pandoc.org/MANUAL.html#extension-shortcut_reference_links)
- [CommonMark](https://spec.commonmark.org/0.29/#shortcut-reference-link)
- [Github Flavored Markdown](https://github.github.com/gfm/#shortcut-reference-link)

syntax/markdown.vim

@@ -94,7 +94,6 @@ syn match markdownRule "- *- *-[ -]*$" contained
 
 syn match markdownLineBreak " \{2,\}$"
 
-syn region markdownIdDeclaration matchgroup=markdownLinkDelimiter start="^ \{0,3\}!\=\[" end="\]:" oneline keepend nextgroup=markdownUrl skipwhite
 syn match markdownUrl "\S\+" nextgroup=markdownUrlTitle skipwhite contained
 syn region markdownUrl matchgroup=markdownUrlDelimiter start="<" end=">" oneline keepend nextgroup=markdownUrlTitle skipwhite contained
 syn region markdownUrlTitle matchgroup=markdownUrlTitleDelimiter start=+"+ end=+"+ keepend contained
@@ -105,6 +104,8 @@ syn region markdownLinkText matchgroup=markdownLinkTextDelimiter start="!\=\[\%(
 syn region markdownLink matchgroup=markdownLinkDelimiter start="(" end=")" contains=markdownUrl keepend contained
 syn region markdownId matchgroup=markdownIdDelimiter start="\[" end="\]" keepend contained
 syn region markdownAutomaticLink matchgroup=markdownUrlDelimiter start="<\%(\w\+:\|[[:alnum:]_+-]\+@\)\@=" end=">" keepend oneline
+syn region markdownShortcutLink matchgroup=markdownLinkTextDelimiter start="!\=\[\%(\_[^][]*]\%( \=[[(]\)\@!\)\@=" end="\]" contains=@markdownInline,markdownLineStart
+syn region markdownIdDeclaration matchgroup=markdownLinkDelimiter start="^ \{0,3\}!\=\[" end="\]:" oneline keepend nextgroup=markdownUrl skipwhite
 
 let s:concealends = ''
 if has('conceal') && get(g:, 'markdown_syntax_conceal', 1) == 1
@@ -131,8 +132,8 @@ for s:type in g:markdown_fenced_languages
   if has_key(s:done_include, matchstr(s:type,'[^.]*'))
     continue
   endif
-  exe 'syn region markdownHighlight_'.substitute(matchstr(s:type,'[^=]*$'),'\..*','','').' matchgroup=markdownCodeDelimiter start="^\s*\z(`\{3,\}\)\s*\%({.\{-}\.\)\='.matchstr(s:type,'[^=]*').'}\=\S\@!.*$" end="^\s*\z1\ze\s*$" keepend contains=@markdownHighlight_'.tr(matchstr(s:type,'[^=]*$'),'.','_') . s:concealends
-  exe 'syn region markdownHighlight_'.substitute(matchstr(s:type,'[^=]*$'),'\..*','','').' matchgroup=markdownCodeDelimiter start="^\s*\z(\~\{3,\}\)\s*\%({.\{-}\.\)\='.matchstr(s:type,'[^=]*').'}\=\S\@!.*$" end="^\s*\z1\ze\s*$" keepend contains=@markdownHighlight_'.tr(matchstr(s:type,'[^=]*$'),'.','_') . s:concealends
+  exe 'syn region markdownHighlight_'.substitute(matchstr(s:type,'[^=]*$'),'\..*','','').' matchgroup=markdownCodeDelimiter start="^\s*\z(`\{3,\}\)\s*\%({.\{-}\.\)\='.matchstr(s:type,'[^=]*').'}\=[^[:space:]:]\@!.*$" end="^\s*\z1\ze\s*$" keepend contains=@markdownHighlight_'.tr(matchstr(s:type,'[^=]*$'),'.','_') . s:concealends
+  exe 'syn region markdownHighlight_'.substitute(matchstr(s:type,'[^=]*$'),'\..*','','').' matchgroup=markdownCodeDelimiter start="^\s*\z(\~\{3,\}\)\s*\%({.\{-}\.\)\='.matchstr(s:type,'[^=]*').'}\=[^[:space:]:]\@!.*$" end="^\s*\z1\ze\s*$" keepend contains=@markdownHighlight_'.tr(matchstr(s:type,'[^=]*$'),'.','_') . s:concealends
   let s:done_include[matchstr(s:type,'[^.]*')] = 1
 endfor
 unlet! s:type
@@ -173,6 +174,7 @@ hi def link markdownLinkText              htmlLink
 hi def link markdownIdDeclaration         Typedef
 hi def link markdownId                    Type
 hi def link markdownAutomaticLink         markdownUrl
+hi def link markdownShortcutLink          markdownId
 hi def link markdownUrl                   Float
 hi def link markdownUrlTitle              String
 hi def link markdownIdDelimiter           markdownLinkDelimiter