$string =~ m/^(frm)(\w+?)(txt)?$/;
There's a problem here in that if 'txt' may not be present (as (txt)? implies) and an overall match is still desired, $3 will be undefined because the third capture group need not match at all. A different approach will yield an empty string instead.
>perl -wMstrict -le
"for (@ARGV) {
m/^(frm)(\w+?)(txt)?$/;
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) ()
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