Index: src/wp-includes/formatting.php
===================================================================
--- src/wp-includes/formatting.php	(revision 28727)
+++ src/wp-includes/formatting.php	(working copy)
@@ -108,9 +108,9 @@
 			$dynamic[ '/(?<=\A|' . $spaces . ')\'(\d[\d\.\,]*)\'/' ] = $opening_single_quote . '$1' . $closing_single_quote;
 		}
 
-		// '99 '99s '99's (apostrophe)
+		// '99 '99s '99's (apostrophe)  But never '9 or '999.
 		if ( "'" !== $apos ) {
-			$dynamic[ '/\'(?=\d)/' ] = $apos;
+			$dynamic[ '/\'(?=\d\d(?:\D|\Z))/' ] = $apos;
 		}
 
 		// Single quote at start, or preceded by (, {, <, [, ", -, or spaces.