/..     # match first two chars
(?:(.)  # capture next char, then
      (?<=    # looking behind,
         (?!\1\1\1) # don't allow a run of three
         ...) # starting three chars back
){1,4}/x

##</code><code>##

/(?<=  # looking behind,
   (?=.{0,3}foo)  # look for a foo preceded by up to three chars
   .{6}) # starting six chars back
 bar/x # then match bar

##</code><code>##

/..     # match first two chars
(?:
  (?<=    # looking behind,
     (?!(.)\1\1) # don't allow a run of three
     ..) # starting only two chars back
 .       # then match the next char
){1,4}/x