We present an online algorithm to deal with pattern matching in strings. The problem we investigate is commonly known as string matching with mismatches in which the objective is to report the number of characters that match when a pattern is aligned with every location in the text. The novel method we propose is based on the frequencies of individual characters in the pattern and the text. The algorithm consumes minimal space in the form of simple arrays, which reduces the cost overhead to maintain the complex data structures such as suffix trees or automaton.