Combine Regexp?

Doesn't contain @: /(^[^@]*$)/

Combining works if the intended result of combination is that any of them matching results in the whole regexp matching.

Will the conditions be ORed or ANDed together?

Starts with: abc
Ends with: xyz
Contains: 123
Doesn't contain: 456

The OR version is fairly simple; as you said, it's mostly a matter of inserting pipes between individual conditions. The regex simply stops looking for a match as soon as one of the alternatives matches.

/^abc|xyz$|123|^(?:(?!456).)*$/

That fourth alternative may look bizarre, but that's how you express "doesn't contain" in a regex. By the way, the order of the alternatives doesn't matter; this is effectively the same regex:

/xyz$|^(?:(?!456).)*$|123|^abc/

The AND version is more complicated. After each individual regex matches, the match position has to be reset to zero so the next regex has access to the whole input. That means all of the conditions have to be expressed as lookaheads (technically, one of them doesn't have to be a lookahead, I think it expresses the intent more clearly this way). A final .*$ consummates the match.

/^(?=^abc)(?=.*xyz$)(?=.*123)(?=^(?:(?!456).)*$).*$/

And then there's the possibility of combined AND and OR conditions--that's where the real fun starts. :D

If a string must not contain @, every character must be another character than @:

/^[^@]*$/

This will match any string of any length that does not contain @.

Another possible solution would be to invert the boolean result of /@/.