\b              # a word boundary
(?!\s)          # following char is not a white space (1)
(?s:            # . matches newline in rest of regex
  .{0,$blen}    # up to $blen chars (left context)
  $rxwords      # followed by a word we search for
  (?:           # group for repeatedly matching
    .{0,$jlen}  #   up to $jlen=$blen+$alen chars (2)
    $rxwords    #   followed by a searched word
  )*            # repeatedly match
  .{0,$alen}    # up to $alen chars (right context)
  (?:           # group for disjunction (3)
    (?<=\s)     #    last matched char was white space
   |            #  or
    [^\s]*      #    non white space chars
    \b          #    up to the next word boundary
  )
)