Allow 6-digit color spec w/o # (#1690)

* Allow 6-digit color spec w/o #

* Improve RegEx pattern

* Pick up comment

* fix lint

src/Parser.js

@@ -743,14 +743,21 @@ export default class Parser {
         if (!res) {
             return null;
         }
-        const match = (/^(#[a-f0-9]{3}|#[a-f0-9]{6}|[a-z]+)$/i).exec(res.text);
+        const match = (/^(#[a-f0-9]{3}|#?[a-f0-9]{6}|[a-z]+)$/i).exec(res.text);
         if (!match) {
             throw new ParseError("Invalid color: '" + res.text + "'", res);
         }
+        let color = match[0];
+        if (/^[0-9a-f]{6}$/i.test(color)) {
+            // We allow a 6-digit HTML color spec without a leading "#".
+            // This follows the xcolor package's HTML color model.
+            // Predefined color names are all missed by this RegEx pattern.
+            color = "#" + color;
+        }
         return newArgument({
             type: "color-token",
             mode: this.mode,
-            color: match[0],
+            color,
         }, res);
     }