Regex date formatting for templates not working - php

Question

I'm trying to replace variables like {{{month}}} in a template to the current month and {{{month+1}}} to current month + 1.
That's not the hardest part of my code, except that the regex I wrote doesn't yield expected results.
$string = '{{{year}}}{{{month+1}}}';
preg_match_all('/{{{(?:([yY])ear|([mM])onth|([dD])ay)(?:(?<operation>[-|+])(?<amount>[1-9]+))?}}}/m', $string, $matches);
var_dump($matches);
Why do I have so much empty array entries?
I was expecting
[0] => array('{{{year}}}', '{{{month+1}}}')
[1] => array('y', 'm')
[2] => array('', '+')
[3] => array('', '1')
What am I doing wrong?
The respond of the above code is:
array(8) {
[0]=>
array(2) {
[0]=>
string(10) "{{{year}}}"
[1]=>
string(13) "{{{month+1}}}"
}
[1]=>
array(2) {
[0]=>
string(1) "y"
[1]=>
string(0) ""
}
[2]=>
array(2) {
[0]=>
string(0) ""
[1]=>
string(1) "m"
}
[3]=>
array(2) {
[0]=>
string(0) ""
[1]=>
string(0) ""
}
["operation"]=>
array(2) {
[0]=>
string(0) ""
[1]=>
string(1) "+"
}
[4]=>
array(2) {
[0]=>
string(0) ""
[1]=>
string(1) "+"
}
["amount"]=>
array(2) {
[0]=>
string(0) ""
[1]=>
string(1) "1"
}
[5]=>
array(2) {
[0]=>
string(0) ""
[1]=>
string(1) "1"
}
}

Answer 1

You may use a "generic" character class to match the first letters of month, year and day, and then use an alternation with positive look-behinds to make sure we match what we need.
preg_match_all('/{{{([yYmMdD])(?:(?<=[Yy])ear|(?<=[Mm])onth|(?<=[Dd])ay)(?:([-‌​+])([1-9]+))?}}}/m', $string, $matches);
See IDEONE demo
And this is the print_r view:
Array
(
[0] => Array
(
[0] => {{{year}}}
[1] => {{{month+1}}}
)
[1] => Array
(
[0] => y
[1] => m
)
[2] => Array
(
[0] =>
[1] => +
)
[3] => Array
(
[0] =>
[1] => 1
)
)