add a RetabSnip command which fixes indentation automatically

autoload/snipMate.vim

@@ -545,5 +545,42 @@ fun! snipMate#GetSnippets(scopes, trigger)
 	return result
 endf
 
+" adds leading tab
+" and replaces leading spaces by tabs
+" see ftplugin/snippet.vim
+fun! snipMate#RetabSnip() range
+  let leadingTab = expand('%:e') == 'snippets'
+
+  let lines = getline(a:firstline, a:lastline)
+
+  " remove leading "\t"
+  let allIndented = 1
+  for l in lines
+	if l[0] != '\t' | let allIndented = 0 | endif
+  endfor
+
+  " retab
+  if allIndented
+	call map(lines, 'v:val[1:]')
+  endif
+
+  let leadingSp = filter(map(copy(lines),'matchstr(v:val,"^\\s*") '),'v:val !=""')
+  if !empty(leadingSp)
+	" lines containing leading spaces found
+	let smallestInd =  len(sort(leadingSp)[-1])
+	let ind = input('retab, spaces per tab: ', smallestInd)
+	for i in range(0, len(lines)-1)
+	  let ml = matchlist(lines[i], '^\(\s*\)\(.*\)')
+	  let lines[i] = repeat("\t", len(ml[1]) / ind)
+				 \ . repeat( " ", len(ml[1]) % ind)
+				 \ . ml[2]
+	endfor
+  endif
+  " readd tab
+  let tab = leadingTab ? "\t" : ""
+  for i in range(0,len(lines)-1)
+	call setline(a:firstline + i, tab.lines[i])
+  endfor
+endf
 
 " vim:noet:sw=4:ts=4:ft=vim

doc/snipMate.txt

@@ -57,8 +57,9 @@ Note that the first hard tab after the snippet trigger is required, and not
 expanded in the actual snippet. The syntax for *.snippet files is the same,
 only without the trigger declaration and starting indentation.
 
-Also note that snippets must be defined using hard tabs. They can be expanded
-to spaces later if desired (see |snipMate-indenting|).
+Also note that indentation within snippets must be defined using hard tabs.
+They can be expanded to spaces later if desired (see |snipMate-indenting|).
+You can retab a snippet by visually selecting the lines, then press <cr>.
 
 "#" is used as a line-comment character in *.snippets files; however, they can
 only be used outside of a snippet declaration. E.g.: >

ftplugin/snippet.vim

@@ -0,0 +1,2 @@
+command -buffer -range=% RetabSnip <line1>,<line2>call snipMate#RetabSnip()
+vnoremap <cr> :RetabSnip<cr>