Using Particle-in-Cell simulations, we study the interaction of few mJ-few cycle laser pulses with an underdense plasma at resonant density. In this previously unexplored regime, it is found that group velocity dispersion is a key ingredient of the interaction. The concomitant effects of dispersion and plasma nonlinearities causes a deceleration of the wakefield phase velocity, which becomes sub-relativistic. Electron injection in this sub-relativistic wakefield is enhanced and leads to the production of a femtosecond electron bunch with picocoulomb of charge in the 5-10 MeV energy range. Such an electron bunch is of great interest for application to ultrafast electron diffraction. In addition, in this dispersion dominated regime, it is shown that positively chirped laser pulses can be used as a tuning knob for compensating plasma dispersion, increasing the laser amplitude during self-focusing and optimizing the trapped charge.