Index: tests/formatting/Slashit.php
===================================================================
--- tests/formatting/Slashit.php	(revision 1095)
+++ tests/formatting/Slashit.php	(working copy)
@@ -4,6 +4,10 @@
  * @group formatting
  */
 class Tests_Formatting_Slashit extends WP_UnitTestCase {
+	function test_backslashes_middle_numbers() {
+		$this->assertEquals("\\a-!9\\a943\\b\\c", backslashit("a-!9a943bc"));
+	}
+
 	function test_backslashes_alphas() {
 		$this->assertEquals("\\a943\\b\\c", backslashit("a943bc"));
 	}

Index: wordpress/wp-includes/formatting.php
===================================================================
--- wordpress/wp-includes/formatting.php	(revision 22313)
+++ wordpress/wp-includes/formatting.php	(working copy)
@@ -1342,8 +1342,18 @@
  * @return string String with backslashes inserted.
  */
 function backslashit($string) {
-	$string = preg_replace('/^([0-9])/', '\\\\\\\\\1', $string);
-	$string = preg_replace('/([a-z])/i', '\\\\\1', $string);
+	// string literals are chosen instead of 0x30..0x39 as value selected
+	// from string is more likely to be string, than integer, thus casted
+	// PHP form shall take place
+	if ( isset( $string[0] ) && $string[0] >= '0' && $string[0] <= '9' ) {
+		$string = '\\\\' . $string;
+	}
+	// Do not use [A..z], as it would also add backslashes to intermediate
+	// characters (namely: '[', '\', ']', '^', '_', '`').
+	$string = addcslashes(
+		$string,
+		'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz'
+	);
 	return $string;
 }