--- /dev/null
+'use strict';
+
+var decode = require('parse-entities');
+
+module.exports = length;
+
+/* Returns the length of HTML entity that is a prefix of
+ * the given string (excluding the ampersand), 0 if it
+ * does not start with an entity. */
+function length(value) {
+ var prefix;
+
+ /* istanbul ignore if - Currently also tested for at
+ * implemention, but we keep it here because that’s
+ * proper. */
+ if (value.charAt(0) !== '&') {
+ return 0;
+ }
+
+ prefix = value.split('&', 2).join('&');
+
+ return prefix.length - decode(prefix).length;
+}