Searching for Anatolian Hieroglyphs in wikitext in the mainspace in English Wiktionary doesn't yield any results, even though there are pages that contain these characters. These are outside the BMP, so perhaps there is some weird bug. However, not all non-BMP characters cause problems.
Searching for Gothic letters (: insource:/[𐌰-𐍊]/) and Egyptian hieroglyphs (: insource:/[𓀀-𓐮]/), which are also outside the BMP, works. I considered the possibility that it might be a Unicode versioning issue, but I'm not sure why regex would refer to the UCD when searching for ranges of code points. (In any case, the Anatolian Supplements block was added in version 8.0, Egyptian Hieroglyphs in 5.2, and Gothic in 3.1.) So I'm mystified as to why some non-BMP characters could be searched for and others couldn't. I wonder if I'm overlooking something obvious here.
Steps to Reproduce:
Enter : insource:/[𔐀-𔙆]/ or : intitle:/[𔐀-𔙆]/ in the search box in English Wiktionary and submit.
No search results.
Both regexes should match the code points in the range U+14400-U+14646 (ANATOLIAN HIEROGLYPH A001 to ANATOLIAN HIEROGLYPH A530) in English Wiktionary. As of the 2019-11-01 dump, the wikitext of two entries contained them – 𒁺𒌓𒋻𒊑𒄿𒀀𒋾𒅖 and 𒋼𒂊𒃷 – and as of the 2019-10-20 dump three entry titles contained them – 𔐱𔕬𔗬𔑰𔖱, 𔑮𔐓𔗵𔗬, 𔖪𔖱𔖪.
I do get one result for insource:/[𔐀-𔙆]/ in all namespaces: Module:scripts/data. That module contains the literal string 𔐀-𔙆. It's as if the regex engine fails to parse [𔐀-𔙆] correctly and instead searches as if the query were insource:/𔐀-𔙆/. That doesn't make sense to me though.