How do I check that a valid email address is entered?
The best these can do is check that it appears to be in a valid format, of course there is no way to check if sending an email to that address will get delivered - short of actually sending a message and checking that there is no bounce!
These tend to look horrendously complicated and so rather than work one out, find one you like off an internet site and go with that.
There are many candidates out there, here is one possibility:
^[^@]{1,64}@[^@]{1,255}$
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