Methods for detecting regularly appearing pattern without excessive memory. The method comprises providing a matrix, and each row of the matrix comprises a location index and a count value. The count value is associated with the location index for indicating the number of the predetermined value that appears at certain locations. After a symbol is received, a current location index representing a current location is translated. A comparing row is picked from the matrix. The matrix is arranged in a circular manner such that the first row is a preliminary comparing row. The count value of the first row is incremented when the current location index equals the location index of the comparing row and the received symbol equals the predetermined value. The received symbol is determined as a regularly appearing pattern according to the count values in the matrix.