fix: preserve selector delimiters after escaped backslashes

src/__tests__/escapes.mjs

@@ -12,6 +12,32 @@ test("escaped semicolon in id", "#\\;", (t, tree) => {
   t.deepEqual(tree.nodes[0].nodes[0].type, "id");
 });
 
+for (const [delimiter, type] of [
+  [".", "class"],
+  ["#", "id"],
+]) {
+  test(`escaped backslash before the ${type} delimiter`, `.foo\\\\${delimiter}bar`, (t, tree) => {
+    t.deepEqual(
+      tree.first.nodes.map(({ type, value }) => [type, value]),
+      [
+        ["class", "foo\\"],
+        [type, "bar"],
+      ],
+    );
+  });
+
+  test(
+    `escaped ${type} delimiter after an escaped backslash`,
+    `.foo\\\\\\${delimiter}bar`,
+    (t, tree) => {
+      t.deepEqual(
+        tree.first.nodes.map(({ type, value }) => [type, value]),
+        [["class", `foo\\${delimiter}bar`]],
+      );
+    },
+  );
+}
+
 // This is a side-effect of allowing media queries to be parsed. Not sure it shouldn't just be an error.
 test("bare parens capture contents as a string", "(h1)", (t, tree) => {
   t.deepEqual(tree.nodes[0].nodes[0].value, "(h1)");

src/parser.js

@@ -105,6 +105,14 @@ function indexesOf(array, item) {
   return indexes;
 }
 
+function isEscaped(word, index) {
+  let escapeStart = index;
+  while (word[escapeStart - 1] === "\\") {
+    escapeStart--;
+  }
+  return (index - escapeStart) % 2 === 1;
+}
+
 function uniqs() {
   // A Set keeps the first-occurrence order the previous filter/indexOf pass
   // produced, without its quadratic cost on long index lists.
@@ -1000,12 +1008,12 @@ export default class Parser {
     }
     const hasClass = indexesOf(word, ".").filter((i) => {
       // Allow escaped dot within class name
-      const escapedDot = word[i - 1] === "\\";
+      const escapedDot = isEscaped(word, i);
       // Allow decimal numbers percent in @keyframes
       const isKeyframesPercent = /^\d+\.\d+%$/.test(word);
       return !escapedDot && !isKeyframesPercent;
     });
-    let hasId = indexesOf(word, "#").filter((i) => word[i - 1] !== "\\");
+    let hasId = indexesOf(word, "#").filter((i) => !isEscaped(word, i));
     // Eliminate Sass interpolations from the list of id indexes
     const interpolations = indexesOf(word, "#{");
     if (interpolations.length) {