Q:

Regex: only alphanumeric but not if this is pure numeric

For instance:
'1'     => NG
'243'   => NG
'1av'   => OK
'pRo'   => OK
'123k%' => NG

I tried with
 /^(?=^[^0-9]*$)(?=[a-zA-Z0-9]+)$/

but it is not working very well. 

A:

Use
/^(?![0-9]*$)[a-zA-Z0-9]+$/

This expression has a negative lookahead to verify that the string is not only numbers.  See it in action with RegExr. 

A:

So we know that there must be at least one "alphabetic" character in there somewhere:
[a-zA-Z]

And it can have any number of alphanumeric characters (including zero) either before it or after it, so we pad it with [a-zA-Z0-9]* on both sides:
/^[a-zA-Z0-9]*[a-zA-Z][a-zA-Z0-9]*$/

That should do the trick. 

A:

Try with this:
/^(?!^\d*$)[a-zA-Z\d]*$/

Edit: since this is essentially the same of the accepted answer, I'll give you something different:
/^\d*[a-zA-Z][a-zA-Z\d]*$/

No lookaheads, no repeated complex group, it just verifies that there's at least one alphabetic character.  This should be quite fast.