Function to convert numeric string to number in version 8.0

Is there a function that will convert a numeric string to a number,
returning zero if the string is empty?

Here is the problem I'm trying to solve; I have a column of type
VARCHAR(4) that usually, but not always, contains a number between 0 and
100, possibly with a decimal point.  I have to do two things;

- Query according to the range of the number, i.e.
   where value >0 and value < 5

Because it's varchar, a number like 17 is between 0 and 5.  For this
particular example I can work around the problem by using
   where value ~ '^ *0*[0-9](\.[0-9]*)* *$' and value > 0 and value < 5
to allow only one digit before the decimal.  (I allowed for
leading/trailing spaces and leading zeros just in case.)

- Sort by that number, i.e.
   order by value

That, I can't work around.  If I use
   order by to_number ( value, '000.99' )
thus adding leading zeros to the test, the order is correct, but the
query fails if there are any empty values.  So I need to either find a
function that converts a numeric string to a float, find a function like
to_number but that doesn't choke on empty values, or write the query
such that my regular expression is combined with to_number.  The c/php
trinary operator would be good here:
   order by ( value ~  '^ *0*[0-9](\.[0-9]*)* *$' ) ?
to_number(value,'000.999) else "";
or something like that, to call to_number only if the string contains a
number, but I don't know the postgresql approach.

If I have to write my own function, any pointers to tutorials?

David