re·gret·ful·ly /rɪˈgrɛtfəli/
(ad.)悔恨地;感到抱歉
Re·gret·ful a. Full of regret; indulging in regrets; repining. -- Re*gret*ful*ly, adv.
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regretfully
adv : with regret (used in polite formulas); "I must regretfully
decline your kind invitation"