docs: improve code samples

Author: kenjis

user_guide_src/source/libraries/file_collections.rst

@@ -107,8 +107,8 @@ retainPattern(string $pattern, string $scope = null)
 Filters the current file list through the pattern (and optional scope), removing
 or retaining matched files.
 
-``$pattern`` may be a complete regex (like ``'#[A-Za-z]+\.php#'``) or a pseudo-regex
-similar to ``glob()`` (like ``*.css``).
+``$pattern`` may be a complete regex (like ``'#\A[A-Za-z]+\.php\z#'``) or a
+pseudo-regex similar to ``glob()`` (like ``'*.css'``).
 
 If a ``$scope`` is provided then only files in or under that directory will be
 considered (i.e. files outside of ``$scope`` are always retained). When no scope