refactor(markdown): simplify regex (#3194)

* refactor(markdown): simplify regex

* fix: check exact string for heading

src/printer-markdown.js

@@ -87,8 +87,8 @@ function genericPrint(path, options, print) {
       const index = parentNode.children.indexOf(node);
       const nextNode = parentNode.children[index + 1];
 
-      // special prefix that may cause different meaning
-      if (nextNode && nextNode.value.match(/^(#{1,6}|>+)|^(-|\+|\*)$/)) {
+      // leading char that may cause different syntax
+      if (nextNode && /^>|^([-+*]|#{1,6})$/.test(nextNode.value)) {
         return node.value === "" ? "" : " ";
       }
 
@@ -104,15 +104,15 @@ function genericPrint(path, options, print) {
           prevNode.type === "sentence" &&
           prevNode.children.length > 0 &&
           prevNode.children[prevNode.children.length - 1].type === "word" &&
-          prevNode.children[prevNode.children.length - 1].value.match(
-            new RegExp(`[^${asciiPunctuationPattern}]$`)
+          new RegExp(`[^${asciiPunctuationPattern}]$`).test(
+            prevNode.children[prevNode.children.length - 1].value
           )) ||
         (nextNode &&
           nextNode.type === "sentence" &&
           nextNode.children.length > 0 &&
           nextNode.children[0].type === "word" &&
-          nextNode.children[0].value.match(
-            new RegExp(`^[^${asciiPunctuationPattern}]`)
+          new RegExp(`^[^${asciiPunctuationPattern}]`).test(
+            nextNode.children[0].value
           ));
       const style =
         hasPrevOrNextWord || getAncestorNode(path, "emphasis") ? "*" : "_";