# \1 = biggest prime factor of N
(?= (xx+) \1* (?!(xx+)\2+$) \1$ )

(?=
        # Main loop!
        (
                # \4 = N % \1, N -= \4
                (x*?) (?=\1*$)

                # \5 = next prime factor of N
                (?= (\4xx+?) (\5* (?!(xx+)\7+$) \5)? $ )

                # \8 = B, \9 = B - 1, \10 = N - B
                (?= ((x*) (?=\5\9*$) x) (\8*) $ )

                x*
                (?=
                        # if \5 = \1, break.
                        (?=\5$) \1
                |
                        # else, N = (\5 - 1) + (N - B)
                        \5\10
                )
                x
        )*
) \10

# Special-case: if N prime, return N-1
| x+\B

# Special-case: if N is 1, return 1
| x