What is the correct way to write a regular expression if the group may not exist?

Question

Regular Expressions

Example strings and desired result:

"11д 17ч 12м" - array(0 => 11, 1 => 17, 2 => 12)
"11д 12м"     - array(0 => 11, 1 => 0,  2 => 12)
"11д 17ч"     - array(0 => 11, 1 => 17, 2 => 0)
"17ч"         - array(0 => 0,  1 => 17, 2 => 0)