An Intriguing Number Pattern Problem

Somebody forwarded this math puzzle several days ago. Believe me it is quite challenging, especially for the math majors.  🙂  To those who want to exercise their brain muscles, you may want to check this out.

8809=6
7111=0
2172=0
6666=4
1111=0
3213=0
7662=2
9313=1
0000=4
2222=0
3333=0
5555=0
8193=3
8096=5
7777=0
9999=4
7756=1
6855=3
9881=5
5531=0
2581=?

By the way, please don’t post the answer.