lateral thinking - Which number does 1995 correspond to in this number pattern?

I saw this puzzle many moons ago and so I assumed it would already be on here somewhere. I have looked and not found, though, and so I present to you a puzzle that baffled me when first I saw it.

What number replaces the question mark in the last example and why?

3531 → 0
3597 → 1
3891 → 3

9397 → 2
3869 → 4
2128 → 2
3786 → 3
3506 → 2
2700 → 2
7212 → 0
7205 → 1
2990 → 3
9503 → 2

1057 → 1
1160 → 2

1995 → ?

Answer

It is

2

You count the amount of

enclosed areas in the numbers