  We have proposed a procedure to extract the probability for valence particle
being out of the binding potential from the measured nuclear asymptotic
normalization coefficients. With this procedure, available data regarding the
nuclear halo candidates are systematically analyzed and a number of halo nuclei
are confirmed. Based on these results we have got a much relaxed condition for
nuclear halo occurrence. Furthermore, we have presented the scaling laws for
the dimensionless quantity $<r^{2}>/R^{2}$ of nuclear halo in terms of the
analytical expressions of the expectation value for the operator $r^{2}$ in a
finite square-well potential.
