tom and (dick or harry or john)

I'll assume you want to match a string containing "tom" and one (or more) of "dick","harry" or "john" in any order.

This can't be done easily in one regular expression, but you can use two (or more):

/tom/ && /dick|harry|john/;
[download]

or

/tom/ && ( /dick/ || /harry/ || /john/ );
[download]

Note that && and || are perl operators, which don't have a corresponding meaning inside a regex. (You can use | in a regex, but there is no & operator, because regexes already match the whole expression by default)