1. With regret.

Nuttall's Standard dictionary of the English language By Nuttall, P.Austin

• regretfull,
• regretfuly,
• regrettfully,
• regreatfully,
• regretablly,
• regetfully,
• regreeffully,
• regaetfull,
• regratfully,
• refresfully,
• eegretfully,
• degretfully,
• fegretfully,
• tegretfully,
• 5egretfully,
• 4egretfully,
• rwgretfully,
• rsgretfully,
• regrestfully,
• regreytfully

• regrettable,
• regretful,
• regrettably,
• re-create.

The adjective form of the word "regretfully" is "regretful".

The plural form of the word "regretfully" is not commonly used. However, if we consider it as an adjective modifying a noun, the plural form would be "regretfuls".

The singular form of the word "regretfully" is "regret"