[DiapDealer]

Sigil is definitely not using any variant of pcre2.

I know of no special meaning for the forward slash in pcre2. I've never needed to escape it. If there's an issue, it would seem to me it is with Regex101 and not Sigil, PCRE2, or Python. It certainly won't hurt to escape them, but it seems a waste of time.

NOTE: the only pcre flavors I see on regex101 seem to be php specific. Am I missing something? Forward slashes would definitely need special treatment if used in php.

My guess is that regex101 being a regex analyzer served by a webserver; the forward slash would most definitely need to be escaped. But to my knowledge, that requirement is not inherent to pcre2 itself.

EDIT: Regex101's PCRE/PCRE2 flavors definitely seem to be predicated on the notion that they're going to be used in PHP. That's a mistake in my opinion, but the extra escapes won't cause any harm if you include them.