The parentheses break the expression into subparts of found text. In this case (paragraph break)(numbers)(paragraph break). You only want to keep the numbers part, so you use the $ notation to tell ID which subpart to keep. $0 is ALL of the found text, $1 is the first group enclosed by parentheses, $2 the second (your numbers), and so on. By choosing $2 you save the numbers, but throw away anything that was found in the first and following subparts.
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