Donald Regan's appearance before the Iran-contra committees yesterday was an inspiration to soreheads everywhere. The former White House chief of staff, fabled in his time for his combative and imperious ways, had turned into a pussycat. Or was doing a good imitation of one.

Members of the committee reacted in roughly the way Londoners did upon beholding the reformed Ebenezer Scrooge on Christmas morning.

"Your appearance here today," Sen. Paul Trible (R-Va.) told Regan, "demonstrates that there is life after Washington."

"It's called Alexandria," Regan said.

"You certainly have mellowed with time," marveled Rep. Dante Fascell (D-Fla.) a bit later. He told Regan he felt he should recommend him "to some winery, to make wonderful wine."

"I'm not sure I get the point," said Regan.

"It was meant as a compliment," said Fascell.

After the rather tepid intransigence of Attorney General Edwin Meese, perhaps as draining and dreary a witness as the committees have entertained (and he certainly didn't entertain them), the new 1987 Donald Regan, with his wry wit and strangely convivial demeanor, reinvigorated the hearings and disarmed the committee members as they haven't been disarmed since Oliver North held them, and the TV cameras, all hostage.

Regan's appearance seemed less a performance than North's was -- but then, John Barrymore's "Hamlet" may have been less a performance than North's, too. Regan was like a man dropping by the office after a long and therapeutic vacation. He was sanguine, carefree, liberated. He was like Maurice Chevalier in "Gigi," when Chevalier sang, "How lovely to sit here in the shade, with none of the woes of man and maid; I'm glad I'm not young anymore."

Clearly, Regan was glad he was not chief of staff anymore. He seemed gladder about it than George Will may be. Watching Regan on TV yesterday, one had to wonder, was this really the cunning, manipulative egomaniac who insisted on being addressed as "chief" by his White House staff and, reportedly, demanded that underlings rise from their seats any time he walked into the room?

Regan made sly references to the way he left the White House and veiled ones to the now famous campaign by Nancy Reagan to oust him. Of Ronald Reagan, Regan said to one questioner, "He's not the type that likes to go around firing people -- ironic statement coming from me."

Later, during a discussion of the proper role of the "fall guy" in political warfare, Regan said, "I don't mind spears in the breast. It's knives in the back that concern me."

Sen. William Cohen (R-Maine) sized up Regan as a man who now didn't have to respond "to anybody in either wing of the White House at this point in your career," a reference to Mrs. Reagan, or at least to Mrs. Reagan's wing. Regan smiled knowingly and said "no comment" to that.

Cohen also thanked Regan for "the touches and flashes of Irish humor" he'd used to punctuate his testimony -- and keep people interested as the hearings face twilight. When Sen. Warren Rudman (R-N.H.) mentioned in passing that he kept a "general diary," for instance, Regan asked jokingly, "Is that available?"

But there was a trace of Irish blarney, too. Referring to the president as "this guy," as Regan did, sounded odd enough, but then he also said of Reagan, "This guy I know was an actor, and he was nominated at one time for an Academy Award."

Perhaps Reagan should have been nominated -- particularly for his acclaimed performance in the 1942 film "Kings Row." But a spokesman for the Academy of Motion Picture Arts and Sciences said from Beverly Hills yesterday that he never was, adding, "and I tend to think he's not going to be in the immediate future."

(North's name was missing from the list of Emmy nominees announced yesterday, too, come to think of it -- the greatest miscarriage of justice since they overlooked Jon Provost for playing Timmy on "Lassie.")

According to "Inside Oscar," by Mason Wiley and Damien Bona, there was some talk of a write-in vote for Reagan on academy ballots in 1942, but the closest Reagan got to an Oscar nomination was an appearance in "Mr. Gardenia Jones," a short that was nominated for best documentary that same year.

So Don Regan isn't an expert on movie trivia. No matter. He still proved adept at charming the pants off the select committees. In part it was his brusque, gloves-off vernacular, something out of David Mamet or maybe "The Untouchables." Regan's descriptions of private White House meetings were not only detailed and evocative (at last -- a witness with a memory!) but revealed a knack for tough-guy storytelling, besides:

President Reagan was so "visibly" shaken when he heard the news of the diversion of Iran arms sale funds to the Nicaraguan contras that, Regan said, "he looked like a guy that really was punched in the stomach."

Handing the president Ollie North's prepared chronology of events pertaining to the arms sale, Regan recalled telling the president of the United States, "Look this over; there's something screwy here."

As the skulduggery of the Iran arms deal progressed, Regan remembered asking at one point, "How many times do we put up with this rug-merchant type of stuff?"

Preparation for the president's ill-fated and error-packed Nov. 19 press conference was a case of too many broths spoiling the cook, Regan suggested. All the conflicting advice, Regan said rather artfully, "sort of confused the presidential mind ... "

Regan was so candid and colorful that he woke everybody up. When Sen. Daniel Inouye (D-Hawaii), the Senate committee chairman, explained early in the morning session that a number of members' seats were vacant because the House was taking a vote, Regan said, "I thought I was boring," which produced one of the day's many rounds of laughter.

This, exulted ABC News White House correspondent Sam Donaldson, was "the quintessential Donald Regan -- blunt, salty," and possessed of an "engaging arrogance." Sam allowed as to how he knew something about engaging arrogance, and that it was a quality journalists were sure to appreciate. "I found him rather engaging, too," Donaldson told anchor Peter Jennings. "Maybe it takes one to know one; I don't know."

Rep. William Broomfield (R-Mich.), when his time came to quiz Regan, congratulated him on testimony that was "absolutely outstanding." The committee likes it when a witness is on a roll. It's good television, and that's good for the committee. Regan was a particular godsend considering the torpor that had nestled over the hearings in their weary waning days.

Regan is scheduled to return to complete his testimony this morning, since his voice, if not his alleged Irish humor, gave out at about 4:30 in the afternoon. The committee members seem so grateful to him, they may greet him with candy and flowers. Don Regan was clearly a hit, and no one seemed inclined to go after him with a big rhetorical club. For one thing, he's clearly well armed on that score.

According to the network rotation plan, yesterday was officially ABC's day to carry the hearings, but executives at NBC News decided they would air Regan, too. "We did this voluntarily," explained Executive Vice President Timothy Russert. "Regan had not been heard from in any meaningful way since he left the White House. I'm glad we did it. We're very pleased."

At about 4:30, NBC cut away from the testimony briefly so that anchor Tom Brokaw could deliver news about the crash of an American helicopter in the Persian Gulf. Today's coverage, the return of Regan and a possible afternoon appearance by Secretary of Defense Caspar Weinberger, will air on NBC, as well as on CNN and PBS.

If Regan is as ingratiating and personable as he was yesterday, it really could give hope to seeming incorrigibles in all walks of life. Who knows but that Sean Penn could turn pacifist, Joan Collins celibate and Raymond Burr anorexic? And Orrin Hatch coherent? Regan said yesterday that one should never lie to Congress but that on occasion he had wanted to tell them "where to go." If he'd done that yesterday, they probably would have gone there.