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Over the last decade, numerous routing and wavelength assignment (RWA) algorithms have been developed for WDM optical networks planning. Most of these algorithms neglect the feasibility of the obtained lightpaths. In this paper, we propose a new algorithm, called LERP (Lightpath Establishment with Regenerator Placement), that enables to solve the problem of RWA in guaranteeing the feasibility of the obtained solution. A lightpath is said admissible if the BER (Bit Error Rate) at its destination node remains acceptable (remains under a given threshold). In the case of BER non-admissibility, one or more electrical regenerators may be placed along the lightpath. The LERP algorithm aims at minimizing the number of regenerators necessary to guarantee the quality of transmission along the lightpath. The originality of the our approach consists in considering simultaneously four physical layer impairments, namely, chromatic dispersion, polarization mode dispersion, amplified spontaneous emission and non-linear phase shift. The efficiency of the LERP algorithm is demonstrated via a numerical comparison with one of the alternative solutions proposed in the literature. Numerical simulations have been carried out in the context of the 18-node NSF network.