An apparatus and a method of operating the apparatus are provided for performing a comparison operation to match a given sequence of values within an input vector. Instruction decoder circuitry is responsive to a string match instruction specifying a segment of an input vector to generate control signals to control the data processing circuitry to perform a comparison operation. The comparison operation determines a comparison value indicative of whether each input element of a required set of consecutive input elements of the segment has a value which matches a respective value in consecutive reference elements of the reference data item. A plurality of comparison operations may be performed to determine a match vector corresponding to the segment of the input vector to indicate the start position of the substring in the input vector. A string match instruction, as well as simulator virtual machine implementations, are also provided.