On further querying and reading, I think this IS a bug. Photon specifies:

"Photon is an ANSI-compliant engine designed to be compatible with modern Apache Spark APIs and just works with your existing code — SQL, Python, R, Scala and Java — no rewrite required."

At least for SQL regexp_replace, this is not the case.

@Suteja Kanuri​ Any response or further thoughts?