Function: multi-isearch-files-regexp

Start multi-buffer regexp Isearch on a list of FILES.
Relative file names in this list are expanded to absolute
file names using the current buffer's value of `default-directory'.
Interactively read file names to search, one by one, ended with RET.
With a prefix argument, ask for a wildcard, and search in file buffers
whose file names match the specified wildcard.