Not sure I follow, but you can build on this:

use warnings;
use strict;

while (<DATA>) {
    chomp;
    if (/ ^ [cljso] \d{4} _ \d*? \. (dat|idx) $ /ix) {
        print "$_ : match\n";
    }
    else {
        print "$_ : no match\n";
    }    
}

__DATA__
C1234ABC.IDX
C1234_ABC.IDX
C1234_.IDX
[download]

prints:

C1234ABC.IDX : no match
C1234_ABC.IDX : no match
C1234_.IDX : match
[download]