optional parameter/value pair

How can I define a route to catch by index controller the urls like:

1. /par1/val1/par2/val2
2. /par2/val2/par1/val1
3. /par1/val1
4. /par2/val2

With legacy urls they would look like :

1. ?par1=val1&par2=val2
2. ?par2=val2&par1=val1
3. ?par1=val1
4. ?par2=val2

1 and 2 are definitelly the same... How can I achieve this behavior with user-friendly urls?

Thanks