1. A content search system configured to determine whether a string of input characters matches a regular expression that includes an activator sub-expression and a counter sub-expression, wherein the activator sub-expression comprises a prefix string and the counter sub-expression comprises a quantified character class, the content search system comprising: an instruction memory for storing a number of instructions, each embodying one of the sub-expressions; a hardware-implemented processor having an input to receive the string of input characters and configured to sequentially process the input characters by executing instructions selectively retrieved from the instruction memory, wherein each input character that matches both the counter sub-expression and the activator sub-expression is classified as an activator character, and each input character that matches the counter sub-expression but not the activator sub-expression is classified as a non-activator character; and a state list for storing a number of state entries that identify which of the instructions stored in the instruction memory are to be executed by the processor to process a current input character, wherein each state entry corresponding to the counter sub-expression includes an address value that identifies a corresponding instruction stored in the instruction memory, a first count value that indicates how many sequential input characters match the counter sub-expression, and a second count value that identifies a position of a first activator to non-activator character transition in the input string.