Google "finite state machine".
Or, read the file one byte at a time, if the byte just doesn't match the first character of the search term, go on to the next byte. If it does match, now you're looking for the next character in the sequence. I.e., your state has gone from 0, to 1. If your state equals (or passes) the length of the search string, you found it!
Implementation/debugging left to the reader.