Leinster's Aviva Stadium dust-up with Scarlets this Saturday will be their third Champions Cup semi-final in four seasons.

If the Welshmen upset the odds, it'll make it a hat-trick of losses at the last-four stage.

They say you learn more about your team, about yourself, in defeat and those setbacks - which both came on French soil, Toulon in 2015, Clermont 12 months ago - left scars. 

Added to that is Scarlets' RDS raid last year in the Pro12 semi-final, a deserved 27-15 success that should act as a red flag to those Leinster fans already checking flights to Bilbao for May's European decider. 

It's certainly etched into the minds of Leo Cullen's players, who are desperate to get over the line and erase some painful memories.

"The experience of playing big games, and losing big games teaches you valuable lessons in sport," said senior coach Stuart Lancaster as prep began for the weekend showdown.

"The challenge last year was obviously going away to Clermont, trying to win away from home against a top team. The experience there was really about the first 15 minutes of the game I felt.

"In the biggest of games you might get something wrong in the first five, ten, 15 minutes and you think it won't happen at the end of the game. But everything does matter, from one to 80 minutes.

"We got two or three things wrong in that first 15 minutes and that cost us two tries and a sin-binning. Suddenly you're chasing the game.

"That was a painful experience. You're learning all the time. The players - particularly the younger players - they've tasted that. They've tasted it in Clermont, they've tasted it in the Pro12 against Scarlets at home.

"We've demonstrated in the pool stage and against Saracens that we can deliver. But that counts for nothing now. Every game starts from zero."

Scarlets come to the capital desperate to spoil the party. Although the prospect of an all-Ireland Champions Cup final between Leinster and Munster has supporters on this island licking their lips, that sort of talk has riled Wayne Pivac's men.

Centre Scott Williams has already admitted their status as underdogs has irritated the camp, going as far as declaring: "If I had £20 spare I’d definitely put it on us beating Leinster and winning the whole thing."

Lancaster isn't siding with the bookies either. He fully expects a pumped-up visiting outfit to make life awkward, as they look to tap into the experience of last year's scalping of Leinster and subsequent hammering of Munster in the final at the Aviva.

A crucial lesson learned, he hopes; particularly for the fresher-faced talents in the panel.

"There's a lot of seasoned players in the Leinster team; there's a lot of young players as well.

"We need to make sure we take all that learning and apply it at the weekend. If a young player is deserving of his opportunity, if he's played and delivered, then you need to trust them.

"I as a coach would always trust a younger player if he'd done the work in the lead-up to the opportunity. The young players at Leinster have shown they can do that."

Scarlets come into this one off the back of a Leigh Halfpenny-inspired slaying of La Rochelle at a packed Parc-y-Scarlets.

Skipper Ken Owens labelled Halfpenny the best goalkicker on the planet afterwards, the full-back's 19 points having paved the way for a memorable win.

It may be Scarlets' first appearance in the semis of the European Cup since 2007, but Lancaster warned their pedigree deserves maximum respect.

"You can't do what they did last year without being a top quality team," he said.

"They are without a doubt the toughest opponent we'll have faced so far in Europe. We're very respectful of what Scarlets will bring." 

Follow Leinster v Scarlets via our live blog on RTÉ.ie and the News Now App (kick-off 3.30pm), or hear live updates on RTÉ Radio 1's Saturday Sport. Follow Racing 92 v Munster (kick-off 3.15pm) via our live blog or listen to live and exclusive radio commentary on RTÉ Radio 1's Sunday Sport.