Users of Arch Linux who build Sigil from scratch may have noticed that the recent upgrade to Python 3.6 forces them to rebuild Sigil (no surprise there). But what they may not have noticed is that it's recommended that any aur-installed Python modules be rebuilt after the Python 3.6 upgrade.
Sigil recommends the regex module be installed to have a complete plugin framework (one the matches the Windows and OS X Sigil implementations), and regex is only available through the AUR on Arch.
Not having regex installed shouldn't break normal Sigil operation, but it may break any plugins that happen to utilize the the regex module.
TL,DR: reinstall those AUR python modules. ;)
Sigil recommends the regex module be installed to have a complete plugin framework (one the matches the Windows and OS X Sigil implementations), and regex is only available through the AUR on Arch.
Not having regex installed shouldn't break normal Sigil operation, but it may break any plugins that happen to utilize the the regex module.
TL,DR: reinstall those AUR python modules. ;)