Win8 Strawberry 5.8.9.5 (32)  Mon 08/16/2021 20:36:57
C:\@Work\Perl\monks
>perl -Mstrict -Mwarnings
use Data::Dump qw(dd);

for my $s (
    '2021-08-16 foo a=bcd bar', '2021-08-17 foo a=bcd',
    '2021-08-18 a=bcd',         '2021-08-19 a=b',
    '2021-08-20 a=',  '2021-08-21 ',  '2021-08-22',  'xyzzy',
    ) {
    my $matched = $s =~
        /(\d\d\d\d-\d\d-\d\d) .*(?:a=([a-z]+))?/   # .* greedy - fails all
      # /(\d\d\d\d-\d\d-\d\d) .*?(?:a=([a-z]+))?/  # .*? lazy - fails some
      # /(\d\d\d\d-\d\d-\d\d) (?:.*a=([a-z]+))?/   # works
      # /(\d\d\d\d-\d\d-\d\d) (?:.*?a=([a-z]+))?/  # works
        ;
    dd $s, $1, $2 if $matched;
  # dd $s, $1, $2;  # ???
    }
^Z
("2021-08-16 foo a=bcd bar", "2021-08-16", undef)
("2021-08-17 foo a=bcd", "2021-08-17", undef)
("2021-08-18 a=bcd", "2021-08-18", undef)
("2021-08-19 a=b", "2021-08-19", undef)
("2021-08-20 a=", "2021-08-20", undef)
("2021-08-21 ", "2021-08-21", undef)

##</code><code>##

>perl -Mstrict -Mwarnings
use Data::Dump qw(dd);

for my $s (
    '2021-08-16 foo a=bcd bar', '2021-08-17 foo a=bcd',
    '2021-08-18 a=bcd',         '2021-08-19 a=b',
    '2021-08-20 a=',  '2021-08-21 ',  '2021-08-22',  'xyzzy',
    ) {
    my $matched = $s =~
      # /(\d\d\d\d-\d\d-\d\d) .*(?:a=([a-z]+))?/   # .* greedy - fails all
        /(\d\d\d\d-\d\d-\d\d) .*?(?:a=([a-z]+))?/  # .*? lazy - fails some
      # /(\d\d\d\d-\d\d-\d\d) (?:.*a=([a-z]+))?/   # works
      # /(\d\d\d\d-\d\d-\d\d) (?:.*?a=([a-z]+))?/  # works
        ;
    dd $s, $1, $2 if $matched;
  # dd $s, $1, $2;  # ???
    }
^Z
("2021-08-16 foo a=bcd bar", "2021-08-16", undef)
("2021-08-17 foo a=bcd", "2021-08-17", undef)
("2021-08-18 a=bcd", "2021-08-18", "bcd")
("2021-08-19 a=b", "2021-08-19", "b")
("2021-08-20 a=", "2021-08-20", undef)
("2021-08-21 ", "2021-08-21", undef)

##</code><code>##

>perl -Mstrict -Mwarnings
use Data::Dump qw(dd);

for my $s (
    '2021-08-16 foo a=bcd bar', '2021-08-17 foo a=bcd',
    '2021-08-18 a=bcd',         '2021-08-19 a=b',
    '2021-08-20 a=',  '2021-08-21 ',  '2021-08-22',  'xyzzy',
    ) {
    my $matched = $s =~
      # /(\d\d\d\d-\d\d-\d\d) .*(?:a=([a-z]+))?/   # .* greedy - fails all
      # /(\d\d\d\d-\d\d-\d\d) .*?(?:a=([a-z]+))?/  # .*? lazy - fails some
        /(\d\d\d\d-\d\d-\d\d) (?:.*a=([a-z]+))?/   # works
      # /(\d\d\d\d-\d\d-\d\d) (?:.*?a=([a-z]+))?/  # works
        ;
    dd $s, $1, $2 if $matched;
  # dd $s, $1, $2;  # ???
    }
^Z
("2021-08-16 foo a=bcd bar", "2021-08-16", "bcd")
("2021-08-17 foo a=bcd", "2021-08-17", "bcd")
("2021-08-18 a=bcd", "2021-08-18", "bcd")
("2021-08-19 a=b", "2021-08-19", "b")
("2021-08-20 a=", "2021-08-20", undef)
("2021-08-21 ", "2021-08-21", undef)