When a buffer visiting the file on disk exists, the matching is performed 
on this buffer: if the buffer is out of sync with the file, the
result may be wrong.

emacs -Q /tmp/foo --eval='(progn (with-current-buffer "foo" (insert "baz")) (save-buffer))'
C-x d RET
M-! printf baz >> bar; for f in foo bar; do printf qux >> $f ; done RET
g
% g \`bazqux$ RET
;; Just match bar.  It may signal an error if trying to read a non regular file.


*) Files satisfying predicate `file-regular-p'.
*) A buffer visiting the file on disk need to be updated before matching the input regexp.

In GNU Emacs 25.0.91.1 (x86_64-unknown-linux-gnu, GTK+ Version 2.24.23)
Repository revision: b1a3ebedba88689d26f44cf7db338104b849ae99