while (<DATA>) {
    if ( /\d{4}-?(\d{3})\s{0,}?(\d\d:\d\d:\d\d)?/ ) {
        print "Match: $_" ;
    } else {
        print "NO MATCH: $_";
    }
}

__DATA__
2005-100 10:12:01
2005100 10:12:01
2005-100
2005100
2005