-syn region htmlBold start=/\(^\|\s\)\*\@<!\*\*\*\@!/ end=/\*\@<!\*\*\*\@!\($\|\s\)/ contains=@Spell,htmlItalic
-syn region htmlItalic start=/\(^\|\s\)\*\@<!\*\*\@!/ end=/\*\@<!\*\*\@!\($\|\s\)/ contains=htmlBold,@Spell
-syn region htmlBold start=/\(^\|\s\)_\@<!___\@!/ end=/_\@<!___\@!\($\|\s\)/ contains=htmlItalic,@Spell
-syn region htmlItalic start=/\(^\|\s\)_\@<!__\@!/ end=/_\@<!__\@!\($\|\s\)/ contains=htmlBold,@Spell
-syn region htmlString start="]("ms=s+2 end=")"me=e-1
-syn region htmlLink start="\["ms=s+1 end="\]"me=e-1 contains=@Spell
-syn region htmlString start="\(\[.*]: *\)\@<=.*" end="$"
+syn region htmlBold start=/\\\@<!\(^\|\A\)\@=\*\@<!\*\*\*\@!/ end=/\\\@<!\*\@<!\*\*\*\@!\($\|\A\)\@=/ contains=@Spell,htmlItalic
+syn region htmlItalic start=/\\\@<!\(^\|\A\)\@=\*\@<!\*\*\@!/ end=/\\\@<!\*\@<!\*\*\@!\($\|\A\)\@=/ contains=htmlBold,@Spell
+syn region htmlBold start=/\\\@<!\(^\|\A\)\@=_\@<!___\@!/ end=/\\\@<!_\@<!___\@!\($\|\A\)\@=/ contains=htmlItalic,@Spell
+syn region htmlItalic start=/\\\@<!\(^\|\A\)\@=_\@<!__\@!/ end=/\\\@<!_\@<!__\@!\($\|\A\)\@=/ contains=htmlBold,@Spell
+
+" [link](URL) | [link][id] | [link][]
+syn region mkdLink matchgroup=mkdDelimiter start="\!\?\[" end="\]\ze\s*[[(]" contains=@Spell nextgroup=mkdURL,mkdID skipwhite oneline
+syn region mkdID matchgroup=mkdDelimiter start="\[" end="\]" contained
+syn region mkdURL matchgroup=mkdDelimiter start="(" end=")" contained
+
+" Link definitions: [id]: URL (Optional Title)
+" TODO handle automatic links without colliding with htmlTag (<URL>)
+syn region mkdLinkDef matchgroup=mkdDelimiter start="^ \{,3}\zs\[" end="]:" oneline nextgroup=mkdLinkDefTarget skipwhite
+syn region mkdLinkDefTarget start="<\?\zs\S" excludenl end="\ze[>[:space:]\n]" contained nextgroup=mkdLinkTitle,mkdLinkDef skipwhite skipnl oneline
+syn region mkdLinkTitle matchgroup=mkdDelimiter start=+"+ end=+"+ contained
+syn region mkdLinkTitle matchgroup=mkdDelimiter start=+'+ end=+'+ contained
+syn region mkdLinkTitle matchgroup=mkdDelimiter start=+(+ end=+)+ contained