There are several ways to express regrets about the past, in other words, to express, "I should have...", "You should have...", "They should have...", etc.. The most formal method uses formula:

imperfect subjunctive of deber + haber + past participle

Debiera haber lavado el coche.
I should have washed the car.

Another method uses the formula:

preterite of deber + haber + past participle

Debí haber hecho la tarea.
I should have done the homework.

Usted debió haber llamado a la policía immediatamente.
You should have called the police immediately.

The following formula may also be used:

imperfect of deber + haber + past participle

Ellos debían haber aceptado que les rebajaran el sueldo.
They should have accepted to have their salary lowered.

Debían haberlos buscado entre los matorrales.
You should have looked for it among the bushes.

The least formal method uses the formula:

imperfect subjunctive of haber + past participle

Hubieras sacado la ropa a secar.
You should have taken out the clothes to dry.

There are many factors that will determine which of these three constructions you should use. There are not only regional differences among the three, but there are differences
in tone as well.