(?# 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 )
    (?=\3*$)
    (?=\2+$)
    \2\4*$
)
\3=\1+$            (?# require that \3 == B, and that N is divisible by \1 )