When attempting to create a new user in piwik, if the user's email address contains an apostrophe (eg firstname.lastname@example.org) Piwik returns "The email doesn't have a valid format." However apostrophes are valid in the local part of email accounts.
Pull request #51 https://github.com/piwik/piwik/commit/35bcaca6abef14b7bdf0c248482537eb04493798 was merged.
Before closing this ticket, uses of email address should be reviewed. For example, in core/Controller.php, an apostrophe appears to break the mailto.
This was fixed almost a year ago. I reviewed uses of email and they are all using double quotes.