regexp match in plpgsql

V8.3.4 on linux

How does one do a regexp match/test in PlPgsql given a text variable containing a regexp and another text variable containt the string to test.  Example that shows what I'm trying to do...

declare

 rgxp text;

 str1 text;

 str2 text;

begin

  rgxp := '[a-zA-Z0-9]';

  str1 := 'ShouldBeOK99';

  str2 := 'Should_Fail_match77';

  if(str1 =~ E\rgxp\) then     raise notice '% is a match',str1;     else    raise notice '% is not a match',str1;    end if;

  if(str2 =~ E\rgxp\) then     raise notice '% is a match',str2;     else    raise notice '% is not a match',str2;    end if;

end;

$$ language plpgsql

I would expect to see...

"ShouldBeOK99 is a match"

"Should_Fail_match77 is not a match"

Thanks in Advance.