  According to heavy-meson chiral perturbation theory, the vector form factor
$f_+(q^2)$ of exclusive semi-leptonic decay $B\to \pi \ell \nu$ is closely
related, at least in the soft-pion region (i.e., $q^{2} \sim (m_B-m_{\pi})^2$),
to the strong coupling $g_{B^* B\pi}$ or the normalized coupling $\hat g$.
Combining the precisely measured $q^2$ spectrum of $B\to \pi \ell \nu$ decay by
the BaBar and Belle collaborations with several parametrizations of the form
factor $f_+(q^2)$, we can extract these couplings from the residue of the form
factor at the $B^*$ pole, which relies on an extrapolation of the form factor
from the semi-leptonic region to the unphysical point $q^2=m_{B^*}^2$.
Comparing the extracted values with the other experimental and theoretical
estimates, we can test these various form-factor parametrizations, which differ
from each other by the amount of physical information embedded in. It is found
that the extracted values based on the BK, BZ and BCL parametrizations are
consistent with each other and roughly in agreement with the other theoretical
and lattice estimates, while the BGL ansatz, featured by a spurious, unwanted
pole at the threshold of the cut, gives a neatly larger value.
