diff --git a/lib/indefinite_article.rb b/lib/indefinite_article.rb
index 46a5fed..4617a1c 100644
--- a/lib/indefinite_article.rb
+++ b/lib/indefinite_article.rb
@@ -4,7 +4,7 @@
 module IndefiniteArticle
 
   A_REQUIRING_PATTERNS = /^(([bcdgjkpqtuvwyz]|onc?e|onearmed|onetime|ouija)$|e[uw]|uk|ubi|ubo|oaxaca|ufo|ur[aeiou]|use|ut([^t])|unani|uni(l[^l]|[a-ko-z]))/i
-  AN_REQUIRING_PATTERNS = /^([aefhilmnorsx]$|hono|honest|hour|heir|[aeiou]|8|11)/i
+  AN_REQUIRING_PATTERNS = /\b11\b|\b18\b|^([aefhilmnorsx]$|hono|honest|hour|heir|[aeiou]|8)/i
   UPCASE_A_REQUIRING_PATTERNS = /^(UN$)/
   UPCASE_AN_REQUIRING_PATTERNS = /^$/ #need if we decide to support acronyms like "XL" (extra-large)