diff --git a/demo/markdownlint-browser.js b/demo/markdownlint-browser.js
index de7bad92..f02ff6aa 100644
--- a/demo/markdownlint-browser.js
+++ b/demo/markdownlint-browser.js
@@ -2750,6 +2750,7 @@ module.exports = {
 // @ts-check
 
 var _a = __webpack_require__(/*! ../helpers */ "../helpers/helpers.js"), addErrorContext = _a.addErrorContext, filterTokens = _a.filterTokens, headingStyleFor = _a.headingStyleFor;
+var closedAtxRe = /^(#+)([ \t]+)([^ \t]|[^ \t].*[^ \t])([ \t]+)(#+)(\s*)$/;
 module.exports = {
     "names": ["MD021", "no-multiple-space-closed-atx"],
     "description": "Multiple spaces inside hashes on closed atx style heading",
@@ -2758,7 +2759,7 @@ module.exports = {
         filterTokens(params, "heading_open", function (token) {
             if (headingStyleFor(token) === "atx_closed") {
                 var line = token.line, lineNumber = token.lineNumber;
-                var match = /^(#+)([ \t]+)([^#]+?)([ \t]+)(#+)(\s*)$/.exec(line);
+                var match = closedAtxRe.exec(line);
                 if (match) {
                     var leftHash = match[1], leftSpaceLength = match[2]["length"], content = match[3], rightSpaceLength = match[4]["length"], rightHash = match[5], trailSpaceLength = match[6]["length"];
                     var left = leftSpaceLength > 1;
diff --git a/lib/md021.js b/lib/md021.js
index 03910930..130daf24 100644
--- a/lib/md021.js
+++ b/lib/md021.js
@@ -5,6 +5,8 @@
 const { addErrorContext, filterTokens, headingStyleFor } =
   require("../helpers");
 
+const closedAtxRe = /^(#+)([ \t]+)([^ \t]|[^ \t].*[^ \t])([ \t]+)(#+)(\s*)$/;
+
 module.exports = {
   "names": [ "MD021", "no-multiple-space-closed-atx" ],
   "description": "Multiple spaces inside hashes on closed atx style heading",
@@ -13,7 +15,7 @@ module.exports = {
     filterTokens(params, "heading_open", (token) => {
       if (headingStyleFor(token) === "atx_closed") {
         const { line, lineNumber } = token;
-        const match = /^(#+)([ \t]+)([^#]+?)([ \t]+)(#+)(\s*)$/.exec(line);
+        const match = closedAtxRe.exec(line);
         if (match) {
           const [
             ,