Understanding lexical grammar for multi-line comments in JavaCC
Let's write this in a more manageable way. I'll use $x$ for any symbol other than $/$ and $*$. Your expression is
$$
/* (/+x)^* * (x(/+x)^** + *)^* /
$$
Now let us apply the following identity to the part "$*(x(/+x)^**+*)^*$":
$$
r(sr)^* = (rs)^*r
$$
We get
$$
/* (/+x)^* (*x(/+x)^*+*)^* */
$$
It's clear what the outer parts $/*$ and $*/$ do, so let's focus on the inner part
$$
(/+x)^* (*x(/+x)^*+*)^*
$$
which captures all strings not containing the substring $*/$. Using the identity
$$
(rs+r)^* = (r^+s)^*
$$
we can simply it to
$$
(/+x)^* (*^+x(/+x)^*)^*
$$
Essentially, each run of stars is "guarded" by a following $x$.