diff --git a/source/tools/autobuild/build.pl b/source/tools/autobuild/build.pl
index 3a6c6daf28..fc49448bcd 100644
--- a/source/tools/autobuild/build.pl
+++ b/source/tools/autobuild/build.pl
@@ -74,7 +74,7 @@ allow_abort();
 $svn_output =~ /^(?:Updated to|At) revision (\d+)\.$/m or die;
 my $svn_revision = $1;
 
-if ($svn_output =~ m~^.  (source(?!/tools)|build|libraries)~)
+if ($svn_output =~ m~^.  (source(?![/\\]tools)|build|libraries)~)
 {
 	# The source has been updated.
 }