A wide set of data obtained on a two-dimensional electron gas submitted to a tunable lateral modulation, induced using a split-gate technique, is presented. Owing to a unique design of the sample, it has been possible to combine in a single experimental run, far-infrared transmission measurements and electrical transport measurements in both directions parallel and perpendicular to the lateral modulation. The discussion of the results emphasizes the correspondence between various features observed in both types of measurements. Based on these features, three regimes of modulation are clearly identified, namely the weak, intermediate and strong modulation regimes. Far-infrared transmission data show that each of these regimes is characterized by plasmon modes with a distinctive behavior. These behaviors are analyzed further with the use of transport data, which allow to determine the electron concentration in the structure for every condition of gate voltage. In the weak modulation regime, a quantitative analysis shows that the collective mode energy is consistent with that of a classical 2D plasmon at q=2 pi/a (where a is the period of the split gate), using the average electron concentration under the gate as the relevant parameter. In the intermediate regime, the collective modes are confined plasmons. The observation of "confined Bernstein modes" indicates that the bare confinement potential is nonparabolic in this regime. In the strong modulation regime, the observation of a far-infrared resonance energy which does not depend on the modulation amplitude, while the effective 2D electron concentration (within each wire) varies with gate voltage, shows that the collective mode is a Kohn mode.