Search and replace in multiple marked files using the “/ Search & Replace›” menu item in casual-dired-tmenu. This will raise a menu of Dired commands that will work on marked files.
While most Dired commands use Emacs-style regular expression syntax ((elisp)Regular Expressions), there are two commands that instead take instead grep-style syntax:
dired-do-find-regexp)
dired-do-find-regexp-and-replace)
The grep-style syntax is dependent on the grep implementation that is installed and used by Emacs.