Haskell - 189 characters
main=interact$r.lines
r f=g t z$last f where{(z:_):t=map words f;g _ s""|s<"["="ACCEPT\n";g([q,j,p]:_)s(i:k)|i:s==j++q=s++' ':i:" -> "++p++'\n':g t p k;g(_:y)s i=g y s i;g _ _ _="REJECT\n"}
EDIT: Does not correctly implement the output for no-transition rejection.
Line-broken version and variable guide:
-- r: run FSM
-- f: fsm definition as lines
-- z: initial state
-- g: loop function
-- t: transition table
-- s: current state
-- i: current input
-- k: rest of input
-- q: transition table match state
-- j: transition table match input
-- p: transition table next state
-- y: tail of transition table
main=interact$r.lines;
r f=g t z$last f where{
(z:_):t=map words f;
g _ s""|s<"["="ACCEPT\n";
g([q,j,p]:_)s(i:k)|i:s==j++q=s++' ':i:" -> "++p++'\n':g t p k;
g(_:y)s i=g y s i;
g _ _ _="REJECT\n"}
I got the s<"[" technique from MtnViewMark's solution; the rest is my own design. Notable characteristics:
- The input is left as junk in the transition table. This is OK as long as the input does not contain two spaces; but note that the transition rule format is arguably unfriendly to transitioning on the space character anyway.
- Stepping through the input string and searching the transition table are the same function.
- Both REJECT cases are handled by the same fallthrough.
