You just have to make parts of the regex optional, and ensure you capture the right parts.To get the captured values as return values, you need to use the /g flag.

#!/usr/bin/perl

while( <DATA> )
    {
    chomp;
    
    my( $year, $julian, $hour, $minute, $second ) = m/
        (\d{4})         # year
        (?:               # group the day - time portions
            -?          # optional hyphen
            (\d{1,3})   # julian day
            (?:           # group the time portions
                \s+     # one or more whitespace
                (\d\d)  # hour
                :
                (\d\d)  # minute
                :
                (\d\d)  # second
            )?            # time is optional
        )?                # day - time is optional
        /xg;
        
    print <<"HERE";
For input [$_]
Year:   $year
Julian: $julian
Hour:   $hour
Minute: $minute
Second: $second

HERE

    }
    
__END__
2005-100 10:12:01
2005100 10:12:01
2005-100
2005100
2005
[download]

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
[download]

/\d{4}-?(\d{3})?\s{0,}?(\d\d:\d\d:\d\d)?/
#              ^--here
[download]

(updated code to fix indenting)

Any suggestions what the correct regexp should look like ?

Here's another approach: you can write (I hope) simple regexps that match each sample datum. It then becomes a simple matter to assemble them together with Regexp::Assemble, and you'll get an efficient pattern as a result:

#! /usr/local/bin/perl -wl

use strict;
use Regexp::Assemble;

print Regexp::Assemble->new->chomp->add(<DATA>)->as_string;

__DATA__
^\d{4}-\d{3} \d\d(?:\d\d){2}$
^\d{4}\d{3} \d\d(?:\d\d){2}$
^\d{4}-\d{3}$
^\d{4}\d{3}$
^\d{4}$
[download]

When I run the above, I get:

update: I forgot to mention... now that you have the pattern, it should be obvious where to put the parentheses to capture the bits you're interested in. That is, as they say, left as an exercise to the reader :)

($year, $julian, $time)=/^(\d{4})-?(\d{3})?\s*(\d{2}:\d{2}:\d{2})?/
[download]

Enjoy,
Mickey