**Corrected version.** Your regular expression allows $/\\#a\color{red}{\\#/}\\#/$, which is explicitly disallowed by the definition: there can be no instance of $/\\#$ between the delimiters, so the red $\\#/$ is not allowed. We need to ensure that no $\\#/$ occurs between the delimiters. To put it a little differently, we need to ensure that if a $\\#$ occurs between the delimiters, it is immediately followed by an $a$, a $b$, another interior $\\#$, or the righthand delimiter.
The regular expression
$$/\\#(a\cup b\cup /\cup \\#a\cup \\#b)^*\\#/$$
almost works, but it’s too restrictive: it doesn’t allow strings of the form $/\\#\\#^+\\#/$, for instance, or strings of the form $/\\#\\#^+a\\#/$, all of which should be acceptable. Two small modifications fix much of the problem:
$$/\\#(a\cup b\cup /\cup \\#^+a\cup \\#^+b)^*\\#/$$
almost works, but it still misses $/\\#\\#\\#/$ and some other legitimate strings. Can you find one more simple fix to patch this hole?