Add a favorite like java.util.regex.Pattern.* to Window > Preferences > Java > Editor > Content Assist > Favorites. Then perform code assist on:

DOT|

Once completed, there are 3 possibilites contingent on the settings indicated below.

import static java.util.regex.Pattern.DOTALL
DOTALL

import java.util.regex.Pattern
Pattern.DOTALL

java.util.regex.Pattern.DOTALL

The middle form is not currently produced when Add Import is checked and Use Static is not.