  We consider numerically the roughness of a planar crack front within the
long-range elastic string model, with a tunable disorder correlation length
$\xi$. The problem is shown to have two important length scales, $\xi$ and the
Larkin length $L_c$. Multiscaling of the crack front is observed for scales
below $\xi$, provided that the disorder is strong enough. The asymptotic
scaling with a roughness exponent $\zeta \approx 0.39$ is recovered for scales
larger than both $\xi$ and $L_c$. If $L_c > \xi$, these regimes are separated
by a third regime characterized by the Larkin exponent $\zeta_L \approx 0.5$.
We discuss the experimental implications of our results.
