You can simplify the specification of a large model when some of your variables have a common prefix by using the colon (:) operator and the dash (-) operator. The dash operator enables you to list variables that are numbered sequentially, and the colon operator selects all variables with a given prefix. For example, if your data set contains the variables X1 through X9, the following MODEL statements are equivalent:

model Y = X1 X2 X3 X4 X5 X6 X7 X8 X9; model Y = X1-X9; model Y = X:;

If your data set contains only the three covariates X1, X2, and X9, then the colon operator selects all three variables:

model Y = X:;

However, the following specification returns an error because X3 through X8 are not in the data set:

model Y = X1-X9;

The double dash (- -) operator enables you to select variables that
are stored sequentially in the SAS data set, whether or not they have a common prefix. You can use the CONTENTS procedure
(see
*Base SAS Procedures Guide*) to determine your variable ordering. For example, if you replace the dash in the preceding MODEL statement with a double
dash, as follows, then all three variables are selected:

model Y = X1--X9;

If your data set contains the variables A, B, and C, then you can use the double dash operator to select these variables by specifying the following:

model Y = A--C;