First of all thanks for replying and for your tips about the ways of writing ([a-z]|[0-9])+. Let's see the rest of your reply:

x) what's about _ and uppercase letters? -> \w instead of a-z0-9

The underscore is captured on item 3:

3) (\-|\_)* : Possibly a dash or underscore.

x) what about user@my-domain.tld ?
x) x.@domain.tld seems ok

Good points. Thanks for pointing them.

BTW: with the flag /x you can write regexes more readable and comment them

True, but I want to make a regex as generic as possible, so I can use it with other langs such as JavaScript and PHP for an example.

Thanks a lot. This was exactly the kind of reply I was looking for. :)

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