A single regex is not the right tool for this, IMO:

/^(?=.*bnano)(?=(?:.*bregulat|.*toxic|(?=.*(?:briskb|bhazardb))(?=.*(?:bexposureb|breleaseb))))/i.test(subject))

would return True if the string fulfills the criteria you set forth, but I find nested lookaheads quite incomprehensible. If JavaScript supported commented regexes, it would look like this:

^                 # Anchor search to start of string

(?=.*bnano)      # Assert that the string contains a word that starts with nano

(?=               # AND assert that the string contains...

 (?:              #  either

  .*bregulat     #   a word starting with regulat

 |                #  OR

  .*toxic         #   any word containing toxic

 |                #  OR

  (?=             #   assert that the string contains

   .*             #    any string

   (?:            #    followed by

    briskb      #    the word risk

   |              #    OR

    bhazardb    #    the word hazard

   )              #    (end of inner OR alternation)

  )               #   (end of first AND condition)

  (?=             #   AND assert that the string contains

   .*             #    any string

   (?:            #    followed by

    bexposureb  #    the word exposure

   |              #    OR

    breleaseb   #    the word release

   )              #    (end of inner OR alternation)

  )               #   (end of second AND condition)

 )                #  (end of outer OR alternation)

)                 # (end of lookahead assertion)

Note that the entire regex is composed of lookahead assertions, so the match result itself will always be the empty string.

Instead, you could use single regexes:

if (/bnano/i.test(str) &&

    ( 

        /bregulat|toxic/i.test(str) ||

        ( 

            /b(?:risk|hazard)b/i.test(str) &&

            /b(?:exposure|release)b/i.test(str)

        )

    )

)    /* all tests pass */