-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="$"
+
+" [link](URL) | [link][id] | [link][]
+syn region mkdLink matchgroup=mkdDelimiter start="\!\?\[" end="\]\ze\s*[[(]" contains=@Spell nextgroup=mkdURL,mkdID skipwhite
+syn region mkdID matchgroup=mkdDelimiter start="\[" end="\]" contained
+syn region mkdURL matchgroup=mkdDelimiter start="(" end=")" contained
+" mkd inline links: protocol optional user:pass@ sub/domain .com, .co.uk, etc optional port path/querystring/hash fragment
+" ------------ _____________________ --------------------------- ________________________ ----------------- __
+syntax match mkdInlineURL /https\?:\/\/\(\w\+\(:\w\+\)\?@\)\?\([A-Za-z][-_0-9A-Za-z]*\.\)\{1,}\(\w\{2,}\.\?\)\{1,}\(:[0-9]\{1,5}\)\?\S*/
+
+" 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