Integrals over subdomains

I have a mesh function which splits my domain into two subdomains, and label them with a 0 and 1 in a mesh function. Next, my variational form is supposed to be something like

a = inner(grad(u),grad(v))*dx(0) + Constant(100)*inner(grad(u),grad(v))*dx(1)

e.g. meaning that subdomain 1 has 100 times higher conductivity. But how do I specify this in the definition of the variatonal problem?

I know that for a boundary integral, say L = v*ds(1), this is done by "exterior_facet_domains = ...", but what is the equivalent for subdomain integrals?