Add onLineContaining ...

...as it will often be easier to test against a particular line by referencing its content rather than figuring out the line number.
In this particular case, I disagree.  The inner class here actually lays out the algorithm in a way that almost makes it read like imperative code.
  1) Initialize the builder and the count
  2) Increment the line number and check for a match
  3) Build the list.

Give that this method doesn't do a whole lot else, keeping the line filtering and the size checking all in sequence makes for the clearest version IMO.