Drawbacks of this approach: if ‘m’ is the length of pattern ‘p’ and ‘n’ the length of string ‘S’, the matching time is of the order O(mn). This is a certainly a very slow running algorithm.
What makes this approach so slow is the fact that elements of ‘S’ with which comparisons had been performed earlier are involved again and again in comparisons in some future iterations. For example: when mismatch is detected for the first time in comparison of p[3] with S[3], pattern ‘p’ would be moved one position to the right and matching procedure would resume from here. Here the first comparison that would take place would be between p[0]=‘a’ and S[1]=‘b’. It should be noted here that S[1]=‘b’ had been previously involved in a comparison in step 2. this is a repetitive use of S[1] in another comparison.
It is these repetitive comparisons that lead to the runtime of O(mn).