I am finally migrating gallery3 today to Piwigo, mainly because flash player was required to upload photos to gallery3 and it is no longer working in Windows 10.

I used the Cpanel editor to edit the config.inc.php and changed the allowable characters...(e.g. adding space, %$ () etc) by the following line:

// permitted characters for files/directories during synchronization
$conf['sync_chars_regex'] = '/^[a-zA-Z0-9-_.()%,$& ']+$/';

however, simulated "synchinization" still does not work.

eg.

[./galleries/Apis-mellifera/Clover-white/DSD_6816(2014-6-7-oh).jpg] PWG-UPDATE-1 (wrong filename)
[./galleries/Apis-mellifera/Clover-white/screenshot_2018-06-15 (1).jpg] PWG-UPDATE-1 (wrong filename)
[./galleries/Apis-mellifera/Clover-white/screenshot_2018-06-15 (6).jpg] PWG-UPDATE-1 (wrong filename)
[./galleries/Apis-mellifera/Clover-white/screenshot_2018-06-16 (1).jpg] PWG-UPDATE-1 (wrong filename)

should I try a different editor?

beepro wrote:

$conf['sync_chars_regex'] = '/^[a-zA-Z0-9-_.()%,$& ']+$/';

You can't use a ' single quote / apostrophe character in that expression. Neither unquoted (which breaks that PHP string assignment anyway) nor quoted. If directories or files contain apostrophe, rename them first.

Including & ampersand isn't a good idea either, it may break in URLs mistakenly interpreted as URL parameters. Similar for % that could be confused with percent escaped hex encoding, or any other URI reserved characters. In general, see this (still unmerged) pull request that would add some comment docs.

beepro wrote:

I wonder if I can maintain the same structure as the old gallery3 site, so that all URL linking to images would still work the same? it might not be possible?

I doubt it's feasible. AFAIR the deeplinks differed by a lot. If you have same album names (and/or possibly set up permalinks in Piwigo) you could redirect like this in your .htaccess:

RedirectMatch permanent /gallery3/var/albums(.*) /piwigo/index?/category$1

And

RedirectMatch permanent /gallery3/(.*) /piwigo/

so it falls back to somewhere instead of giving a plain 404.

with gallery2, I was able to download the original photo after logging in, and click on each photo and download it (no water mark and original uploaded size).

it seems this is not possible with Piwigo?  if I want to go the backend to retrieve the original not-watermarked photo, where do I go?

another feature of gallery2 was to display
number of albums
number of photos,
number of visits
for the home page and each subdir also. I am not sure how to do this in Piwigo?

Admin -> Dashboard and Admin -> Tools -> History.
For more detailed statistics use [extension by grum] AStat (needs [extension by grum] Grum Plugin Classes). It may also help to use [extension by plg] No Stats For Robots along, and use [extension by Eric] Prune History to prune history to a sensible amount, otherwise AStat may get very slow after some weeks.
Prune History needs a small patch https://github.com/Eric-Piwigo/Prune_History/pull/1 that doesn't get merged as upstream appears to be dead.

Your Piwigo gallery seems to be much much faster to me compared to Gallery2  (This has been my experience with myself moving from G2 to Piwigo)

It does, if you take a look at the files changed view. It's plugins/Prune_History/include/functions.inc.php where you change line 185 from
AND date > "'.$startdate.';';
to

AND date > "'.$startdate.'"';

i.e. replace the one second-last ; semicolon with a " double quote character.
(the end of file line feed change is insignificant).

Or cd into plugins/Prune_History directory and run

curl https://github.com/Piwigo/Prune_History/commit/4320e2b6d671cf4d08f13458e6f1df2c4cc2f050.patch | patch -p1

I don't know why that pull request is still not merged even that Prune_History is a repository under the main Piwigo account and thus whether its development is dead or not shouldn't really matter. Apparently some Piwigo maintainers are not keen to merge fixes (this is not the only one that is still pending) or don't follow what's coming in and is pending.