What is the difference between "as to ~" and "regarding ~"?
"As to" means "with respect to" or "concerning," and "regarding" (or "with/in regard to") is a preposition with a similar meaning. There is no notable difference between the two and they can be used interchangeably, as long as you make sure the verb tense is correct. However, the speaker in this video says, "so lucky as to fall in love with my best friend," with the "so lucky as to" being a different expression than "as to." Thus, we can understand "regarding" as also being inappropriate in this context.
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