An expression like this:
[a-zA-Z]*[0-9\+\*][a-zA-Z0-9\+\*]*
should work just fine (obviously insert any additional special characters you want to allow or use ^ operator to match anything except letters/numbers); no need to use complicated lookarounds. This approach makes sense if you only want to allow a certain subset of special characters that you know are "safe", and disallow all others.
If you want to include all special characters except certain ones which you know are "unsafe", then it makes sense to use something like:
\w[^\\]*[^a-zA-Z\\][^\\]*
In this case, you are explicitly disallowing backslashes in your password and allowing any combination with at least one non-alphabetic character otherwise.
The expression above will match any string containing letters and at least one number or +,*. As for the "length of 8" requirement, theres really no reason to check that using regex.