#!/usr/bin/perl

use strict; # https://perlmonks.org/?node_id=11158577
use warnings;

for ( split /\n/, <<END )
#foo bli bla
bli #foo bar baz
#more than #one is a #string
more than #one is a #string
foo bli bla
# does not match #
END
  {
  /(?|#(\S+)|()\z)/ and print "found '$1' in '$_'\n";
  }

##</code><code>##

found 'foo' in '#foo bli bla'
found 'foo' in 'bli #foo bar baz'
found 'more' in '#more than #one is a #string'
found 'one' in 'more than #one is a #string'
found '' in 'foo bli bla'
found '' in '# does not match #'