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    <p>Here's a non-technical answer from Microsoft:</p> <p>DFA engines run in linear time because they do not require backtracking (and thus they never test the same character twice). They can also guarantee matching the longest possible string. However, since a DFA engine contains only finite state, it cannot match a pattern with backreferences, and because it does not construct an explicit expansion, it cannot capture subexpressions.</p> <p>Traditional NFA engines run so-called "greedy" match backtracking algorithms, testing all possible expansions of a regular expression in a specific order and accepting the first match. Because a traditional NFA constructs a specific expansion of the regular expression for a successful match, it can capture subexpression matches and matching backreferences. However, because a traditional NFA backtracks, it can visit exactly the same state multiple times if the state is arrived at over different paths. As a result, it can run exponentially slowly in the worst case. Because a traditional NFA accepts the first match it finds, it can also leave other (possibly longer) matches undiscovered.</p> <p>POSIX NFA engines are like traditional NFA engines, except that they continue to backtrack until they can guarantee that they have found the longest match possible. As a result, a POSIX NFA engine is slower than a traditional NFA engine, and when using a POSIX NFA you cannot favor a shorter match over a longer one by changing the order of the backtracking search.</p> <p>Traditional NFA engines are favored by programmers because they are more expressive than either DFA or POSIX NFA engines. Although in the worst case they can run slowly, you can steer them to find matches in linear or polynomial time using patterns that reduce ambiguities and limit backtracking.</p> <p>[http://msdn.microsoft.com/en-us/library/0yzc2yb0.aspx]</p>
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    1. PODFA vs NFA engines: What is the difference in their capabilities and limitations?
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    1. COThe MSDN article is quite misleading; NFAs and DFAs are equally powerful. NFA algorithms do not require backtracking (which has worst case exponential behavior). The reason backtracking is needed is because "regular expressions" are much more powerful (e.g. backreferences) than regular languages and so they can't be modeled by the canonical NFA/DFAs. Example of well-implemented NFA algorithm that doesn't use backtracking: http://swtch.com/~rsc/regexp/regexp1.html
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