[dba-SQLServer]How to determine whether string is a number?

I need to determine whether the string '4d4' is number or not. But it 
appears that the ISNUMERIC function is not always accurate as shown below:

SELECT ISNUMERIC(SUBSTRING('4D4',1,3));

It should evaluate to 0 but it returns 1! You can try for yourself.

Basically, I need a way to determine that '4D4' is not a number. Anyone have 
any ideas?

Thanks in advance,
Billy

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