Pulled from https://parsoid-vs-core.wmflabs.org/diff/dsbwiki/Welar
Ex.
[[Test|[123]]]
Extlink tokenizing skips the Utils::isProtocolValid check in the presence of an entity (the skip is really supposed to be for the presence of templates but isn't trying hard enough) so
λ (master) echo "[123]]" | php bin/parse.php --trace peg
0-[peg] | ----> [{"type":"SelfclosingTagTk","name":"extlink","attribs":[{"k":"href","v":["123",{"type":"TagTk","name":"span","attribs":[{"k":"typeof","v":"mw:Entity"}],"dataParsoid":{"src":"]","srcContent":"]","tsr":[4,4]}},"]",{"type":"EndTagTk","name":"span","attribs":[],"dataParsoid":{"tsr":[9,9]}}],"srcOffsets":[1,1,1,9]},{"k":"mw:content","v":"","srcOffsets":[9,9,9,9]},{"k":"spaces","v":""}],"dataParsoid":{"tsr":[0,10]}}]and then we bail on the extlink in the token transforms. Unfortunately, that's too late to give back the closing ] to match the wikilink above.