$string =~ m/^(frm)(\w+?)(txt)?$/;

##</code><code>##

>perl -wMstrict -le
"for (@ARGV) {
   m/^(frm)(\w+?)(txt)?$/;
   print qq{'$_' ($1) ($2) ($3)};
   }
 print '-----------';
 for (@ARGV) {
   m/^(frm)(\w+?)(txt|)$/;
   print qq{'$_' ($1) ($2) ($3)};
   }
"  frmNametxt  frmNameFoo
'frmNametxt' (frm) (Name) (txt)
Use of uninitialized value $3 in concatenation (.) or string at ...
'frmNameFoo' (frm) (NameFoo) ()
-----------
'frmNametxt' (frm) (Name) (txt)
'frmNameFoo' (frm) (NameFoo) ()