How to verify a value is a number?

I have a list of values and some might contain symbols, other might contain letter. See example below. How can I flag a value when it's not a number? Column a is the original column. Column B is the new column where only when column a is a number then column b will also be populated with the same value as column a. If column a is not a number or contain symbols or letter then column b will be blank. Will appreciate any inputs! Thank you!

ab
1010
0.1230.123
> 100 
100 ul 
< 200 
563.25563.25
1245.131245.13
10001000
10.0909090910.09090909
-1191-1191
N/A 
~4 ml