fix #544 use sed for improving highting performance

doc/vim-markdown.txt

@@ -287,6 +287,19 @@ Fenced code block languages ~
 <
   Default is "['c++=cpp', 'viml=vim', 'bash=sh', 'ini=dosini']".
 
+                                            *g:vim_markdown_highlight_with_sed*
+- 'g:vim_markdown_highlight_with_sed'
+
+  Vim-markdown needs to search all fenced code blocks for the filetypes to
+  highlight. The default code uses vim's builtin regex engine which is slow
+  dealing with large markdown documents. Enabling this option would use
+  command line utility 'sed' for searching. It's expected to improve performance 
+  around 10x.
+>
+  let g:vim_markdown_highlight_with_sed = 1
+<
+  Default is '0'.
+
 -------------------------------------------------------------------------------
                                             *vim-markdown-follow-named-anchors*
 Follow named anchors ~

ftplugin/markdown.vim

@@ -787,10 +787,19 @@ function! s:MarkdownHighlightSources(force)
     " Syntax highlight source code embedded in notes.
     " Look for code blocks in the current file
     let filetypes = {}
-    for line in getline(1, '$')
-        let ft = matchstr(line, '```\s*\zs[0-9A-Za-z_+-]*\ze.*')
-        if !empty(ft) && ft !~ '^\d*$' | let filetypes[ft] = 1 | endif
-    endfor
+
+    if get(g:, 'vim_markdown_highlight_with_sed', 0)
+        let l:ft_lines = systemlist('sed -n ' . "'" . '/^[[:space:]]*```[[:space:]]*\([0-9A-Za-z_+-]*\).*$/s//\1/p' . "'", bufnr())
+        for ft in l:ft_lines
+            if !empty(ft) && ft !~ '^\d*$' | let filetypes[ft] = 1 | endif
+        endfor
+    else
+        for line in getline(1, '$')
+            let ft = matchstr(line, '```\s*\zs[0-9A-Za-z_+-]*\ze.*')
+            if !empty(ft) && ft !~ '^\d*$' | let filetypes[ft] = 1 | endif
+        endfor
+    endif
+
     if !exists('b:mkd_known_filetypes')
         let b:mkd_known_filetypes = {}
     endif