We report a measurement of the branching fraction of the decay $B rightarrow D^{(*)}pi ell u$. The analysis uses 772$times 10^6$ $Bbar{B}$ pairs produced in $e^+e^-rightarrow Upsilon(4S)$ data recorded by the Belle experiment at the KEKB asymmetric-energy $e^+e^-$ collider. The tagging $B$ meson in the decay is fully reconstructed in a hadronic decay mode. On the signal side, we reconstruct the decay $B rightarrow D^{(*)}pi ell u$ $(ell=e,mu)$. The measured branching fractions are $mathcal{B}(B^+ rightarrow D^-pi^+ ell^+ u)$ = [4.55 $pm$ 0.27 (stat.) $pm$ 0.39 (syst.)]$times 10^{-3}$, $mathcal{B}(B^0 rightarrow bar{D}^0pi^- ell^+ u)$ = [4.05 $pm$ 0.36 (stat.) $pm$ 0.41 (syst.)]$times 10^{-3}$, $mathcal{B}(B^+ rightarrow D^{*-}pi^+ ell^+ u)$ = [6.03 $pm$ 0.43 (stat.) $pm$ 0.38 (syst.)]$times 10^{-3}$, and $mathcal{B}(B^0 rightarrow bar{D}^{*0}pi^- ell^+ u)$ = [6.46 $pm$ 0.53 (stat.) $pm$ 0.52 (syst.)]$times 10^{-3}$. These are in good agreement with the current world average values.
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