php - What's the rule for preg_match() setting an empty string or not setting? -

consider following code:

preg_match('/(\d)(\d?)/', '1', $matches); var_export($matches); 

it outputs:

array (   0 => '1',   1 => '1',   2 => '', ) 

so optional [2] entry set, empty string.

however, remember in past have seen cases optional capturing group, when used last, not set (the array stop @ [1]).

did dream, or there cases entry can not set?

i know use if (! isset($matches[2]) || $matches[2] == '') if want sure, clutter code needlessly.

it depends on whether make whole capturing group optional or not.

if make group required allow match empty string match set, might empty:

preg_match('/(\d)(\d?)/', '1', $matches); // $matches[2] === '' 

if make group optional moving quantifier ? outside brackets, match set conditionally:

preg_match('/(\d)(\d)?/', '1', $matches); // no $matches[2]