12-07-2011 04:24 PM
The tilde (~) is a "logical operator" that means "not". It is synonymous with the "^" symbol.
"." is the missing value for a numeric variable.
"IF <expression>;" is called a "subsetting IF" statement.
So, the statement means
IF pregp IS NOT EQUAL TO MISSING then continue (otherwise, read in another observation and start over at the beginning of the DATA step).
You can find out more about the logical operators and the subsetting IF statement format at support.sas.com by searching for those terms in the documentation.