SQL Server's isNumeric() equivalent in amazon redshift

Try something like:

where field1 ~ '^[0-9]{4}'

It will match any string, that starts with 4 digits.

Although long time has passed since this question was asked I have not found an adequate response. So I feel obliged to share my solution which works fine on my Redshift cluster today (March 2016).

The UDF function is:

create or replace function isnumeric (aval VARCHAR(20000))
  returns bool
IMMUTABLE 
as $$
    try:
       x = int(aval);
    except:
       return (1==2);
    else:
       return (1==1);
$$ language plpythonu;

Usage would be:

select isnumeric(mycolumn), * from mytable
    where isnumeric(mycolumn)=false

It looks like the code you are looking for the is the similar to function:

where left(field,4) similar to '[0-9]{4}'

Redshift doc