The delta-regime of QCD is characterized by light quarks in a small spatial box, but a large extent in the temporal direction. In this setting a specific variant of chiral perturbation theory (the delta expansion) applies, based on a quantum mechanical treatment of the quasi-one-dimensional system. In particular, for vanishing quark masses we obtain a residual pion mass, which has been computed to the third order in the delta expansion. A comparison with a numerical measurement of this residual mass allows for a new the determination of low energy constants. We first show that direct simulations in the delta-regime are very difficult. However, an extrapolation of the pion masses measured in a larger volume (p-regime) towards the delta-regime leads to good agreement with the theoretical predictions. From these results, we extract values for the pion decay constant and for a sub-leading low energy constant.