Field operator does not accept a regex

``` $ gawk 'BEGIN{$0=3; print $/3/}' 3 $ nawk 'BEGIN{$0=3; print $/3/}' nawk: syntax error at source line 1 context is BEGIN{$0=3; print >>> $/ <<< nawk: illegal statement at source line 1 nawk: illegal statement at source line 1 ``` This should be legal according to POSIX. I can't find explicit support for it in the TAPL book (2nd ed.) but it seems to be at least implied.