Following some discussion on the Games Workshop community website pages it has become apparent that some clarification of the 'And they shall know no fear' rule, most beloved of Space Marine commanders, is required.
The rule in Codex Space Marines gives Space Marines four major advantages over other troops:
1. They may regroup even if they are below half strength.
2. If an enemy sweeping advance contacts them they regroup automatically. Lesser troops have to risk a regroup test in these circumstances and are destroyed if they fail.
3. They do not have to take a test to regroup, instead doing it automatically.
4. They regroup at the end of their fall back move rather than at the start of their next move so get back into action quicker.
To be able to regroup, however, there are three criteria. The unit needs to be:
1. In coherency.
2. Not below 50% strength.
3. Outside 6" of any enemy.
Whilst 'And they shall know no fear' lets you ignore (2) it does not help with (1) and (3), so if they are to regroup the Space Marines must be in a coherent formation (ie, within 2" of each other) and have no enemy within 6" of them. If this is not the case then they will continue to fall back. Remember this is not an abject rout, the Space Marines are simply falling back to a less exposed position in accordance with their doctrine and training.
Last chance regrouping at the edge of the table also requires that the regrouping criteria are met, so if the unit is out of coherency or has enemy within 6" it will retreat straight off the table. This may be bad news for you as the player but it saves Space Marines for the inevitable clinical counter-attack and prevents loss of precious gene-seed.
Regrouping in cover is similarly unaffected; Space Marines will automatically regroup if eligible to do so, though, and don't have to risk the vagaries of a Morale test.
It has been pointed out to me that it is possible that a Space Marine unit might find itself prevented from regrouping at the end of its fall back by an enemy unit which subsequently moves further away or is shot down. The Space Marines will therefore find themselves able to regroup at the start of their next move without having to fall back further. If so they will regroup immediately and may move and/or shoot normally.
Remember that Space Marines regroup at the end of their fall back move. According to the sequence of play for assault on page 62 of the Warhammer 40,000 rulebook, fall back moves are performed before sweeping advances and consolidations. This means that any enemy unit which tries to pursue Space Marines (either by sweeping or consolidating) falling back from close combat will not do it quickly enough to prevent them regrouping if the Space Marines fall back 7" or more. If the enemy advance into base contact, the Space Marines will regroup automatically as mentioned above.
It has been said that the best way to take advantage of this rule is to carefully shepherd the Space Marines so that they fall back off-table. This really is one of those 'fine in theory' assertions. Even though the Space Marines are falling back they can still fire at enemies within 12", which means they will be able to fire at any enemy unit that is close enough to stop them regrouping. Also the rest of the Space Marine army may not wish to co-operate with the ploy and the presence of even one supporting unit should be enough to make close pursuit very risky. In practice even when Space Marines have elected to fall back, pursuing them is a dangerous proposition and over-complication is a sure route to disaster.
Rather more significant is that a Space Marine unit that is subject to crossfire before completing a fall back and regrouping will be destroyed. Even Space Marines will sometimes realise too late that their path to safety is blocked, and a moment of confusion is all that their enemies need.