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.