1. A computer system for detecting presence of a subject string S in a target string T of alphanumeric characters, where T has a length m, the computer system comprising: a processor circuit; and a memory in communication with the processor circuit, wherein the memory stores instructions that when executed by the processor circuit cause the processor circuit to determine a similarity score indicative of a similarity between the subject string S and the target string T by: for each of one or more substrings U of target string T, correlating a frequency of occurrence of a set C of n unique alphanumeric characters between the subject string S and the one or more substrings U, where n≦m; normalizing a result of the correlation to produce a score for each of the one or more substrings U; determining the similarity score indicative of the similarity between the subject string S and the target string T by selecting a greatest score from the scores for each of the one or more substrings U; and at least partially responsive to selecting the greatest score, triggering a response to the subject string S in the target string T.