diff --git a/components/prism-markup.js b/components/prism-markup.js
index 9c074efdd8..73fe302d81 100644
--- a/components/prism-markup.js
+++ b/components/prism-markup.js
@@ -1,7 +1,7 @@
 Prism.languages.markup = {
 	'comment': /<!--[\w\W]*?-->/,
 	'prolog': /<\?[\w\W]+?\?>/,
-	'doctype': /<!DOCTYPE[\w\W]+?>/,
+	'doctype': /<!DOCTYPE[\w\W]+?>/i,
 	'cdata': /<!\[CDATA\[[\w\W]*?]]>/i,
 	'tag': {
 		pattern: /<\/?(?!\d)[^\s>\/=$<]+(?:\s+[^\s>\/=]+(?:=(?:("|')(?:\\\1|\\?(?!\1)[\w\W])*\1|[^\s'">=]+))?)*\s*\/?>/i,