Index: _source/core/ckeditor_basic.js
===================================================================
--- _source/core/ckeditor_basic.js	(revision 5395)
+++ _source/core/ckeditor_basic.js	(working copy)
@@ -188,7 +188,7 @@
 					// The textarea class name could be passed as the function
 					// parameter.
 
-					var classRegex = new RegExp( '(?:^| )' + arguments[0] + '(?:$| )' );
+					var classRegex = new RegExp( '(?:^|\\s)' + arguments[0] + '(?:$|\\s)' );
 
 					if ( !classRegex.test( textarea.className ) )
 						continue;