auto-mode-alist and regular expression

[Decebal]

Because files with names like 'adm[a-z]{3}' (admapp, admenv,
admchk, ...) really are shell-scripts, I have in my auto-mode-alist:
         ("/adm[a-z]*\\'"   . shell-script-mode)

But I would like something like:
         ("/adm[a-z]{3}\\'"   . shell-script-mode)

The problem is that this not work.
Offcourse I could do something like:
         ("/adm[a-z][a-z][a-z]\\'"   . shell-script-mode)

But I would prefer a neater solution. Is this possible?