1. A computer implemented method of comparing a first representation of an identifier with a second representation of an identifier, the method comprising: converting the first representation to an expression of the first representation; generating one or more variant expressions from the expression of the first representation; converting the second representation to an expression of the second representation; providing a plurality of other representations and converting each of the other representations to an expression of the other representations; generating one or more variant expressions from each expression of the plurality of other expressions; defining a boundary with respect to the expression of the second representation, the boundary defining a space inside the boundary; determining a first value, the first value being an expression of the number of the variant expressions of the expression of the first representation within the space defined by the boundary; determining a second value, the second value being an expression of the number of variant expressions of the expressions of the other representations within the space defined by the boundary; determining a measure of comparison between the first representation of the identifier and the second representation of the identifier from the first value and the second value.