Is there an informix equivalent to the function isnumeric() ?

I don't think there is one, but you can create it very easily
using someting like

LET charstr = integer value

FOR i = 1 to LENGTH(charstr)
IF charstr[i] in (1,2,3,4 etc... ) THEN
-- do nothing
ELSE
ERROR "Not a numberic value"
RETURN FALSE
END IF
END FOR

I did not try it, Do not have access to 4gl but should work

This is what we use:

drop function isnumeric;
create function isnumeric(inputstr char(12)) returning integer;
define numeric_var decimal(12,6);
define function_rtn integer;

on exception in (-1213)
let function_rtn = 0;
end exception with resume

let function_rtn = 1;
let numeric_var = inputstr;

return function_rtn;
end function;


Fuction returns a "1" if numeric, "0" if non-numeric. I'm just trapping the numeric conversion error.