#my $re = qr{(?x: (?<![/-]) \b ( [0-9]{3,4} ) \b (?![/-]) )};
my $re = qr{(?ax: (?<!\d) (\d{3,4}) (?!\d) )};

##</code><code>##

1..10
ok 1 - Testing: 12
ok 2 - Testing: 123
ok 3 - Testing: 1234
ok 4 - Testing: 12345
ok 5 - Testing: 123 4567 890
ok 6 - Testing: 123 4567 89
not ok 7 - Testing: 123-4567-890
#   Failed test 'Testing: 123-4567-890'
#   at ./pm_1178784_regex_digits.pl line 29.
#          got: '>123<>123<->4567<>4567<->890<>890<'
#     expected: '123-4567-890'
not ok 8 - Testing: 01/02/2017
#   Failed test 'Testing: 01/02/2017'
#   at ./pm_1178784_regex_digits.pl line 29.
#          got: '01/02/>2017<>2017<'
#     expected: '01/02/2017'
not ok 9 - Testing: 2017-01-02T17:01:34
#   Failed test 'Testing: 2017-01-02T17:01:34'
#   at ./pm_1178784_regex_digits.pl line 29.
#          got: '>2017<>2017<-01-02T17:01:34'
#     expected: '2017-01-02T17:01:34'
ok 10 - Testing: 12
# 345
# 6789
# 0
# Looks like you failed 3 tests of 10.