Function testing if a string is a number ?

Hi list !

I am searching for a function that would return true if a string
is a number.
In Access I would have written IsNumeric('mystring'), but this
function does not exist in postgres.
Maybe I am doing this the wrong way actually : I want to fill an
integer field with the values from a varchard field. Some of the
values are not convertible though, so just executing :
UPDATE thetable
SET numfield=varcharfield::int4;
returns an error 'Invalid input syntax for integer "blablabla"'.
My idea was to run :
UPDATE thetable
SET numfield=varcharfield::int4
WHERE IsNumeric(varcharfield);
but maybe there is a better way to achieve this ?

Thanks for your help !

Regards
--
Arnaud