SQL function equivalent to TestN?

Is there anything in SQL that can test a string for a valid numeric?
I am looking to extract the minimum value from a character column in selected rows, but I only want to consider those where the column has a numeric value.
Simplified, I'm looking for something like:

Exec sql
select min(columnOfwhoKnowsWhat)
from table1
where [columnOfwhoKnowsWhat (a 10-character field) only contains some whole number between 1 and 99,999 (possibly with a comma) ];

The column I need to test is a number of feet from a location, but being a "miscellaneous" field, may instead have all manner of other data instead, including values with more than one decimal point, e.g. ".057 .034 .032" (yeah, there's even an embedded space).

I know I can declare a cursor, fetch each row and test with RPG, but if SQL can do it, that would be preferable.

Had to ask...
Thanks.

Michael Koester