(?# Match correct multiplication statements in the domain ^x+\*x*=x*$ using only the ECMAScript subset of regex functionality )
(?# A = first factor; B = second factor; N = number we're testing as the potential product of A and B )
^
(x(x*)).           (?# \1 = A; \2 = A-1 )
(?=
    x*=            (?# skip over B )
    (x?(x*))       (?# \3 = N / \1; \4 = \3-1, or 0 if \3==0 )
    (?=\3*$)
    (
        (?=\2+$)
        \2\4*$
    |
        $\3
    )
)
\3=\1*$            (?# require that \3 == B, and that N is divisible by \1 )