You didn't say which (if any) of the three data records is supposed to yield "yep"... it looks like none of them will, because $VALID3 specifies letters only, and all three data lines have only digits in the third field. Also, for any of them to match, $PST should include "\s*" after the close bracket, as well as before it (or maybe this should be added before the open bracket in $PRE).

You do have the right notion for converting a literal (two character) '\n' or '\t' into the corresponding regex for the given type of whitespace.

Note that some portions of your regexes can be simplified:  [a-zA-Z0-9_] is really just "\w", and if you want to match space, newline and tab, you might as well just use "\s".

Are $VALID1 and $VALID2 really supposed to accept periods and asterisks, as well as alphanumerics and whitespace? (Just checking... sometimes people tend to make the mistake of putting ".*" inside of square brackets when they really have something else in mind.)