https://example.com/Documentation/Product????/index.html
You say it's "not a query string", but actually that is exactly what it is. And that is why you can't match it with the RewriteRule pattern. The above URL is split as follows:
- URL-path:
/Documentation/Product (matched by the RewriteRule pattern)
- Query string:
???/index.html (note 3 ? - the first one starts the query string)
To match the query string you'll need an additional RewriteCond directive that checks against the QUERY_STRING server variable.
For example, to match the above URL, you would need to do something like:
RewriteCond %{QUERY_STRING} ^\?*/index\.html
RewriteRule ^Documentation/Product$ https://s3.amazonaws.com/company-documentation/Help/Product/index.html [NC,R,L]
This matches any number of erroneous ? at the start of the query string.
I've added the R (redirect) flag. Your directive (without the R flag) would trigger an external redirect anyway (because you specifying an absolute URL in the substitution), but it is far better to be explicit here. This is also a temporary (302) redirect. If this should be permanent (301) then change it to R=301, but only once you have confirmed that it's working OK (301s are cached hard by the browser so can make testing problematic).
UPDATE:
...so I want the rule above to match both question marks and any other character.
Only if there are question marks in the URL will there be a query string, so I think it is advisable to keep these two rules separate.
If there could be any erroneous characters at the start of the query string and if you want to capture the end part of the URL (like you are doing in your original directive, eg. index.html) then you can modify the above to read:
RewriteCond %{QUERY_STRING} /(.*)$
RewriteRule ^Documentation/Product$ https://s3.amazonaws.com/company-documentation/Help/Product/%1 [NC,R,L]
Note the %1 (as opposed to $1) backreference in the substitution string. This is a backreference to the captured group in the last matched CondPattern (ie. /(.*)$).
You can follow this with your existing directive (but remember to include the R flag) for more "normal" URLs that don't contain a ? (ie. query string).
NB: Surrounding the arguments in double quotes are entirely optional in this example. They are only required if you have unescaped spaces in the pattern or substitution arguments.
In summary
# Redirect URLs of the form:
# "/Documentation/Product?<anything#1>/<anything#2>"
RewriteCond %{QUERY_STRING} /(.*)$
RewriteRule ^Documentation/Product$ https://s3.amazonaws.com/company-documentation/Help/Product/%1 [NC,R,L]
# Redirect URL-paths of the form (no query string):
# "/Documentation/Product<something>/<anything>"
RewriteRule ^Documentation/Product[^/]+/(.*) https://s3.amazonaws.com/company-documentation/Help/Product/$1 [NC,R,L]
