I personally have no idea how SEC filings are formatted, let alone how they usually discuss deaths and death benefits. In general, this sort of question can be best presented by giving us samples of what you do what to match off and what you don't. See How do I post a question effectively?. And please wrap code in <code> tags.

Based upon what you've written, I'm going to assume you require a regular expression that matches the phrases deceased, passed away, died and dying as well as death but not death benefits. You appear to have had the right concept by wanting to use a negative look-ahead. Meeting this spec might look like:

/\b(?:deceased|passed\s+away|died|dying|death(?!\s+benefits))\b/i

which YAPE::Regex::Explain describes as

The regular expression:

(?i-msx:\b(?:deceased|passed\s+away|died|dying|death(?!\s+benefits))\b
+)

matches as follows:
  
NODE                     EXPLANATION
----------------------------------------------------------------------
(?i-msx:                 group, but do not capture (case-insensitive)
                         (with ^ and $ matching normally) (with . not
                         matching \n) (matching whitespace and #
                         normally):
----------------------------------------------------------------------
  \b                       the boundary between a word char (\w) and
                           something that is not a word char
----------------------------------------------------------------------
  (?:                      group, but do not capture:
----------------------------------------------------------------------
    deceased                 'deceased'
----------------------------------------------------------------------
   |                        OR
----------------------------------------------------------------------
    passed                   'passed'
----------------------------------------------------------------------
    \s+                      whitespace (\n, \r, \t, \f, and " ") (1
                             or more times (matching the most amount
                             possible))
----------------------------------------------------------------------
    away                     'away'
----------------------------------------------------------------------
   |                        OR
----------------------------------------------------------------------
    died                     'died'
----------------------------------------------------------------------
   |                        OR
----------------------------------------------------------------------
    dying                    'dying'
----------------------------------------------------------------------
   |                        OR
----------------------------------------------------------------------
    death                    'death'
----------------------------------------------------------------------
    (?!                      look ahead to see if there is not:
----------------------------------------------------------------------
      \s+                      whitespace (\n, \r, \t, \f, and " ")
                               (1 or more times (matching the most
                               amount possible))
----------------------------------------------------------------------
      benefits                 'benefits'
----------------------------------------------------------------------
    )                        end of look-ahead
----------------------------------------------------------------------
  )                        end of grouping
----------------------------------------------------------------------
  \b                       the boundary between a word char (\w) and
                           something that is not a word char
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------
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See Looking ahead and looking behind in perlretut.