The more informed people would actually know that the SAS recruit RAF Regt.
Selection is held twice a year regardless of conditions. A candidate must be male and have been a regular member of the Armed Forces for at least three years or a member of 21 SAS or 23 SAS (which can be joined directly from civilian life) for at least 18 months. All soldiers who apply must have at least 39 months of military service remaining. A candidate who fails any stage of the selection is 'Returned to [his parent] Unit' (RTU'd). Candidates are allowed only two attempts at selection, after which they may never reapply. Many are not even allowed that.
Like other sections of the British armed forces, the SAS accepts members from the Commonwealth and The Republic of Ireland, with notable representation from Fiji, the former Rhodesia, New Zealand and Australia. The Parachute Regiment is the SAS's main recruiting area.
The selection phase can be broken down in to three main sections: 1) Physical endurance 2) Combat 3) Survival and evading capture
The physical endurance phase is the phase in which most candidates will be eliminated due to the difficulty of the test. The SAS use the Brecon Beacons in Wales to carry out the test. The candidates must under go a long distance hike, and a short distance, but timed, run. For the long distance test, the candidates will carry about 50-80lbs of weight including a rifle. The short distance run is usually 8 miles, but the candidates must complete this in an hour.
The combat phase is held in the jungle, normally in Africa or Asia. This is where the successful candidates will learn to use weapons and tactics to outwit and outmanouver enemy forces. The SAS, unlike most regiments, uses live ammunition on their combat phase. This is because they are trained to fight while considering friendly positions from the start, so that they can carry this straight on to the battlefield: 'You only get one chance'.
If candidates have managed to pass through this combat phase then now they have to go through survival and escape and evasion training. In this phase candidates that are left from the hundreds that apply will undergo a survival phase in the jungle, in which they only have a small 'survival kit'. They must 'survive' for a week while evading an enemy tracking party. This is a particularly hard phase because the tracking party is normally accustomed to the ground. After this week, the candidates must give themselves up at an agreed meeting point. They will then be taken back to the enemy HQ and interrogated. This interrogation phase will make or break their career as they must under go physical and mental torture as well as aggressive interrogation. The SAS will roughly accept 2-7% of the soldiers that started selection.
Personnel completing selection are placed on probation for 4 years and undergo specialist and continuation training appropriate to their employing Troop or more general training such as language. This training will include Jungle, Desert, Urban and Counter-terrorism specialist courses.
I hope that answers your question..