alas vs regrettably

alas

noun
  • A type of depression which occurs in Yakutia, formed by the subsidence of permafrost. 

intj
  • Used to express sorrow, regret, compassion or grief. 

regrettably

adv
  • In a manner inspiring or deserving regret. 

How often have the words alas and regrettably occurred in a corpus of books? (source: Google Ngram Viewer )