Fix issue in MD020/no-missing-space-closed-atx that reported errors for single-character headers (fixes #229).

lib/md020.js

@@ -13,7 +13,7 @@ module.exports = {
     forEachLine(lineMetadata(), (line, lineIndex, inCode) => {
       if (!inCode) {
         const match =
-          /^(#+)(\s*)([^#]+?[^#\\])(\s*)((?:\\#)?)(#+)(\s*)$/.exec(line);
+          /^(#+)(\s*)([^#]*?[^#\\])(\s*)((?:\\#)?)(#+)(\s*)$/.exec(line);
         if (match) {
           const [
             ,

test/short-headings.md

@@ -0,0 +1,83 @@
+# Short Headings
+
+<!-- markdownlint-disable single-title heading-style -->
+
+#A
+
+{MD018:5}
+
+# B
+
+#  C
+
+{MD019:11}
+
+#D#
+
+{MD020:15}
+
+# E #
+
+#  F  #
+
+{MD021:21}
+
+##G
+
+{MD018:25}
+
+## H
+
+##  I
+
+{MD019:31}
+
+##J##
+
+{MD020:35}
+
+## K ##
+
+##  L  ##
+
+{MD021:41}
+
+#MM
+
+{MD018:45}
+
+# NN
+
+#  OO
+
+{MD019:51}
+
+#PP#
+
+{MD020:55}
+
+# QQ #
+
+#  RR  #
+
+{MD021:61}
+
+##SS
+
+{MD018:65}
+
+## TT
+
+##  UU
+
+{MD019:71}
+
+##VV##
+
+{MD020:75}
+
+## WW ##
+
+##  XX  ##
+
+{MD021:81}