A parser is used for parsing a header in a packet. The parser includes a plurality of horizontal field selectors, a plurality of comparators, and a content addressable memory (CAM) based device. Each of the horizontal field selectors is configured to select a first bit group. The comparators are coupled to the horizontal field selectors, respectively. Each of the comparators is configured to compare a first bit group selected by a corresponding horizontal field selector with a designated value to generate a comparison result. The CAM based device is configured to receive a plurality of comparison results generated from the comparators, and use the comparison results as a first input search data.