i just remembered something: whenever !$identifier is used as an operand in a condition, it isn't actually treated as plaintext. it has the effect of logically reversing the value of $identifier in the following way, and in the given order:

• $true becomes $false
• $false becomes $true
• 0 becomes 1
• any number becomes 0
• $null becomes $true
• any other strings becomes $null

here's an example:

//if (!$null = !$(abc)) { } | echo -a $v1 -- $v2

notice the empty string ($null) becomes $true, and the value 'abc' became $null.

thus if ($2 = !$isalnum) doesn't quite check the value of $2 is literally '!$isalnum' but it is still a far cry away from checking that $2 is not alphanumeric.