How can you get in the caves of the regi's do you need dig?

Yes, you need Dig, Dive, Surf, Relicanth, and Wailord.

Yes, you need dig. You can get the TM Dig from the brother of the Fossil Maniac, in his house in Fallabor(sorry about spelling). You then go to Pacifidlog and surf along the currents. If you surf in the right places, you should end up floating above a small space where you can dive. Dive down and head downwards to the other end of the cavern and go in front of the braille writing, where you can surface. Get out of the water and head through the entrance. There should be walls everywhere with braille writing on them. Head to the top of the room and stand in front of the braille. Then use Dig. You'll end up in another cavern. Head towards the top of the room. For this part you need a Wailord at the back of your party, and a Relicanth at the front. Then read the braille writing. Afterwards there should be a small earthquake, and the sound of three doors opening. You have now unlocked the Regis. Regirock is located in the south part of the desert in a small cave. You need to go to the top of the room inside the cave, then walk down 2 steps, then walk 3 steps right. Then use strength. A door will appear. Go inside and catch Regirock. Registeel is found in a cave in the south of route 120. Go inside and go to the middle of the room. Use fly. A door to Registeel will appear. To get Regice, surf down route 104 and 105. While surfing down you will see 2 islands. Surf between them and through the gaps in the rocks. You should find a small island with a cave. Go inside the cave and go up to the braille writing. Press A twice, then wait. Don't do anything. Eventually the door to Regice will open.
Hope this helped!

No, you need to have a Wailord and Relicanth to be able to unlock the Regi caves.

First, capture a Relicanth underwater on Route 124 or 126, and obtain a Wailord by fishing on Route 29 or evolving Wailmer at level 40. Next, go to Pacifidlog Town and head west. Go along the bottom of the current until you hit Route 134, then go across the sandbar that separates Routes 133 and 134. You'll see a diving area. Go down there, proceed until you find a white mound, and resurface in front of the mound's inscription.

In the Sealed Chamber, go to the inscription at back and use Dig. (You can get the TM28 from the little by in the front room of the Fossil Maniac's House. Trapinch learns Dig at level 41, Nincada learns it at level 45, and Diglett learns it at level 17.) Go into the next chamber, put Wailord in the leading spot in your party and put Relicanth in the final position, and read the inscription at back. A tremor should occur and three doors should be opening.

Regirock's cave is in the southern part of the Desert on Route 111 (go in, read the inscription, walk two steps left, walk two steps down, and then use Rock Smash to get access to Regirock), Regice's cave is on an island on the western side of Route 105 (go in, read the inscription, and run a full lap along the perimeter of the chamber to get access to Regice), and Registeel's cave is on the western side of Route 120 (go in, use Flash in the centre of the room, and proceed to the next chamber to fight Registeel).

No, it is a much complicated way.
You better go see in a Walkthrough.

To get in the caves you need a wailord in the first place in your party and a relicanth in the last place.

Yes you do need dig

Okay the dude with the million word answer is right about regirock and steel but regiice you walk around the room once starting from the middle braille thing and go around the room without leaving the wall i'm a girl so I know everything