[yaml mode] Tweak key regexp

Issue #2695

mode/yaml/yaml.js

@@ -80,7 +80,7 @@ CodeMirror.defineMode("yaml", function() {
       }
 
       /* pairs (associative arrays) -> key */
-      if (!state.pair && stream.match(/^\s*[^\-:{}"\[\]][^:{}"\[\]]*(?=\s*:($|\s))/i)) {
+      if (!state.pair && stream.match(/^\s*(?:[,\[\]{}&*!|>'"%@`][^\s'":]|[^,\[\]{}#&*!|>'"%@`])[^#]*?(?=\s*:($|\s))/)) {
         state.pair = true;
         state.keyCol = stream.indentation();
         return "atom";