If the patterns contain captures, then 
/pat1/ && /pat2/

[download]
and 
/(?=.*?pat1)(?=.*?pat2)/s

[download]
aren't equivalent in the first place.

Note also that the OP didn't specify whether pat1 and pat2 may overlap. /pat1/ && /pat2/ has the potential to match more than /pat1.*pat2|pat2.*pat1/s. All of the presented solutions so far assume it's ok the patterns may overlap.

In reply to Re^4: AND and OR on Regular Expressions by JavaFan
in thread AND and OR on Regular Expressions by vitoco

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