Section 4.6. NFA, DFA, and POSIX

4.6. NFA, DFA, and POSIX

4.6.1. "The Longest-Leftmost "

Let me repeat what I've said before: when the transmission starts a DFA engine from some particular point in the string, and there exists a match or matches to be found at that position, the DFA finds the longest possible match, period. Since it's the longest from among all possible matches that start equally furthest to the left, it's the "longest-leftmost" match.

4.6.1.1. Really, the longest

Issues of which match is longest aren't confined to alternation . Consider how an NFA matches the (horribly contrived) one(self)?(selfsufficient)? against the string oneselfsufficient . An NFA first matches one and then the greedy (self)? , leaving (selfsufficient)? left to try against sufficient . It doesnt match, but that's OK since it is optional. So, the Traditional NFA returns sufficient and discards the untried states. (A POSIX NFA is another story that we'll get to shortly.)

On the other hand, a DFA finds the longer . An NFA would also find that match if the initial (self)? were to somehow go unmatched, as that would leave (selfsufficient)? then able to match. A Traditional NFA doesnt do that, but the DFA finds it nevertheless, since it's the longest possible match available to the current attempt. It can do this because it keeps track of all matchessimultaneously, and knows at all times about all possible matches.

I chose this silly example because it's easy to talk about, but I want you to realize that this issue is important in real life. For example, consider trying to match continuation lines . It's not uncommon for a data specification to allow one logical line to extend across multiple real lines if the real lines end with a backslash before the newline. As an example, consider the following:

 SRC=array.c builtin.c eval.c field.c gawkmisc.c io.c main.c \             missing.c msg.c node.c re.c version.c 

You might normally want to use ^\w+=.* to match this kind of "var = value assignment line, but this regex doesn't consider the continuation lines. (I'm assuming for the example that the tool's dot won't match a newline.) To match continuation lines, you might consider appending (\\ \n .*)* to the regex, yielding ^\w+=.* . Ostensibly, this says that any number of additional logical lines are allowed so long as they each follow an escaped newline. This seems reasonable, but it will never work with a traditional NFA. By the time the original .* has reached the newline, it has already passed the backslash , and nothing in what was added forces it to backtrack (˜ 152). Yet, a DFA finds the longer multiline match if available, simply because it is, indeed, the longest.

If you have lazy quantifiers available, you might consider using them, such as with ^\w+=. *? (\\ \n. *? )* . This allows the escaped newline part to be tested each time before the first dot actually matches anything, so the thought is that the \\ then gets to match the backslash before the newline. Again, this wont work. A lazy quantifier actually ends up matching something optional only when forced to do so, but in this case, everything after the = is optional, so theres nothing to force the lazy quantifiers to match anything. Our lazy example matches only ' SRC= ', so it's certainly not the answer.

There are other approaches to solving this problem; we'll continue with this example in the next chapter (˜ 186).

4.6.2. POSIX and the Longest-Leftmost Rule

The POSIX standard requires that if you have multiple possible matches that start at the same position, the one matching the most text must be the one returned.

The POSIX standard document uses the phrase "longest of the leftmost." It doesn't say you have to use a DFA, so if you want to use an NFA when creating a POSIX tool, what's a programmer to do? If you want to implement a POSIX NFA, you'd have to find the full and all the continuation lines, despite these results being " unnatural " to your NFA.

A Traditional NFA engine stops with the first match it finds, but what if it were to continue to try options (states) that might remain ? Each time it reached the end of the regex, it would have another plausible match. By the time all options are exhausted, it could simply report the longest of the plausible matches it had found. Thus, a POSIX NFA.

An NFA applied to the first example would, in matching (self)? , have saved an option noting that it could pick up matching one(self)? (selfsufficient)? at one selfsufficient . Even after finding the sufficient that a Traditional NFA stops at, a POSIX NFA continues to exhaustively check the remaining options, eventually realizing that yes, there is a way to match the longer (and in fact, longest) .

In Chapter 7, we'll see a method to trick Perl into mimicking POSIX semantics, having it report the longest match (˜ 335).

4.6.3. Speed and Efficiency

If efficiency is an issue with a Traditional NFA (and with backtracking, believe me, it can be), it is doubly so with a POSIX NFA since there can be so much more backtracking. A POSIX NFA engine really does have to try every possible permutation of the regex, every time. Examples in Chapter 6 show that poorly written regexes can suffer extremely severe performance penalties.

4.6.3.1. DFA efficiency

The text-directed DFA is a really fantastic way around all the inefficiency of backtracking. It gets its matching speed by keeping track of all possible ongoing matches at once. How does it achieve this magic?

The DFA engine spends extra time and memory when it first sees the regular expression, before any match attempts are made, to analyze the regular expression more thoroughly (and in a different way) from an NFA. Once it starts actually attempting a match, it has an internal map describing "If I read such-and-such a character now, it will be part of this-and-that possible match." As each character of the string is checked, the engine simply follows the map.

Building that map can sometimes take a fair amount of time and memory, but once it is done for any particular regular expression, the results can be applied to an unlimited amount of text. It's sort of like charging the batteries of your electric car. First, your car sits in the garage for a while, plugged into the wall, but when you actually use it, you get consistent, clean power.

NFA: Theory Versus Reality

The true mathematical and computational meaning of "NFA" is different from what is commonly called an "NFA regex engine." In theory, NFA and DFA engines should match exactly the same text and have exactly the same features. In practice, the desire for richer, more expressive regular expressions has caused their semantics to diverge. An example is the support for backreferences .

The design of a DFA engine precludes backreferences, but it's a relatively small task to add backreference support to a true (mathematically speaking) NFA engine. In doing so, you create a more powerful tool, but you also make it decidedly nonregular (mathematically speaking). What does this mean? At most, that you should probably stop calling it an NFA, and start using the phrase "nonregular expressions," since that describes (mathematically speaking) the new situation. No one has actually done this, so the name "NFA" has lingered, even though the implementation is no longer (mathematically speaking) an NFA.

What does all this mean to you, as a user ? Absolutely nothing. As a user, you don't care if it's regular, nonregular, unregular, irregular, or incontinent. So long as you know what you can expect from it (something this chapter shows you), you know all you need to care about.

For those wishing to learn more about the theory of regular expressions, the classic computer-science text is chapter 3 of Aho, Sethi, and Ullman's CompilersPrinciples, Techniques, and Tools (Addison-Wesley, 1986), commonly called "The Dragon Book" due to the cover design. More specifically , this is the "red dragon." The "green dragon" is its predecessor, Aho and Ullman's Principles of Compiler Design .

The work done when a regex is first seen (the once-per-regex overhead) is called compiling the regex . The map-building is what a DFA does. An NFA also builds an internal representation of the regex, but an NFA's representation is like a mini program that the engine then executes.

4.6.4. Summary: NFA and DFA in Comparison

Both DFA and NFA engines have their good and bad points.

4.6.4.1. DFA versus NFA: Differences in the pre-use compile

Before applying a regex to a search, both types of engines compile the regex to an internal form suited to their respective match algorithms. An NFA compile is generally faster, and requires less memory. There's no real difference between a Traditional and POSIX NFA compile.

4.6.4.2. DFA versus NFA: Differences in match speed

For simple literal-match tests in "normal" situations, both types match at about the same rate. A DFA's match speed is generally unrelated to the particular regex, but an NFA's is directly related .

A Traditional NFA must try every possible permutation of the regex before it can conclude that there's no match. This is why I spend an entire chapter (Chapter 6) on techniques to write NFA expressions that match quickly. As we'll see, an NFA match can sometimes take forever. If it's a Traditional NFA, it can at least stop if and when it finds a match.

A POSIX NFA, on the other hand, must always try every possible permutation of the regex to ensure that it has found the longest possible match, so it generally takes the same (possibly very long) amount of time to complete a successful match as it does to confirm a failure. Writing efficient expressions is doubly important for a POSIX NFA.

In one sense, I speak a bit too strongly, since optimizations can often reduce the work needed to return an answer. We've already seen that an optimized engine doesn't try ^ -anchored regexes beyond the start of the string (˜ 149), and well see many more optimizations in Chapter 6.

The need for optimizations is less pressing with a DFA since its matching is so fast to begin with, but for the most part, the extra work done during the DFA's pre-use compile affords better optimizations than most NFA engines take the trouble to do.

Modern DFA engines often try to reduce the time and memory used during the compile by postponing some work until a match is attempted. Often, much of the compile-time work goes unused because of the nature of the text actually checked. A fair amount of time and memory can sometimes be saved by postponing the work until it's actually needed during the match. (The technobabble term for this is lazy evaluation .) It does, however, create cases where there can be a relationship among the regex, the text being checked, and the match speed.

4.6.4.3. DFA versus NFA: Differences in what is matched

A DFA (or anything POSIX) finds the longest leftmost match. A Traditional NFA might also, or it might find something else. Any individual engine always treats the same regex/text combination in the same way, so in that sense, it's not "random," but other NFA engines may decide to do slightly different things. Virtually all Traditional NFA engines I've seen work exactly the way I've described here, but it's not something absolutely guaranteed by any standard.

4.6.4.4. DFA versus NFA: Differences in capabilities

An NFA engine can support many things that a DFA cannot. Among them are:

• Capturing text matched by a parenthesized subexpression. Related features are backreferences and after-match information saying where in the matched text each parenthesized subexpression matched.

• Lookaround, and other complex zero-width assertions ^{[ ]} (˜ 133).

  ^{[ ]} lex has trailing context , which is exactly the same thing as zero-width positive lookahead at the end of the regex, but it can't be generalized and put to use for embedded lookahead .

• Non-greedy quantifiers and ordered alternation. A DFA could easily support a guaranteed shortest overall match (although for whatever reason, this option never seems to be made available to the user), but it cannot implement the local laziness and ordered alternation that we've talked about.

• Possessive quantifiers (˜ 142) and atomic grouping (˜ 139).

DFA Speed with NFA Capabilities: Regex Nirvana?

I've said several times that a DFA can't provide capturing parentheses or backreferences. This is quite true, but it certainly doesn't preclude hybrid approaches that mix technologies in an attempt to reach regex nirvana. The sidebar on page 180 told how NFAs have diverged from the theoretical straight and narrow in search of more power, and it's only natural that the same happens with DFAs. A DFA's construction makes it more difficult, but that doesn't mean impossible .

GNU grep takes a simple but effective approach. It uses a DFA when possible, reverting to an NFA when backreferences are used. GNU awk does something similarit uses GNU grep 's fast shortest-leftmost DFA engine for simple "does it match" checks, and reverts to a different engine for checks where the actual extent of the match must be known. Since that other engine is an NFA, GNU awk can conveniently offer capturing parentheses, and it does via its special gensub function.

Tcl's regex engine is a true hybrid, custom built by Henry Spencer (whom you may remember having played an important part in the early development and popularization of regular expressions ˜ 88). The Tcl engine sometimes appears to be an NFAit has lookaround, capturing parentheses, backreferences, and lazy quantifiers. Yet, it has true POSIX longest-leftmost match (˜ 177), and doesn't suffer from some of the NFA problems that we'll see in Chapter 6. It really seems quite wonderful.

4.6.4.5. DFA versus NFA: Differences in ease of implementation

Although they have limitations, simple versions of DFA and NFA engines are easy enough to understand and to implement. The desire for efficiency (both in time and memory) and enhanced features drives the implementation to greater and greater complexity.

With code length as a metric, consider that the NFA regex support in the Version 7 (January 1979) edition of ed was less than 350 lines of C code. (For that matter, the entire source for grep was a scant 478 lines.) Henry Spencer's 1986 freely available implementation of the Version 8 regex routines was almost 1,900 lines of C, and Tom Lord's 1992 POSIX NFA package rx (used in GNU sed, among other tools) is a stunning 9,700 lines.

For DFA implementations , the Version 7 egrep regex engine was a bit over 400 lines long, while Henry Spencer's 1992 full-featured POSIX DFA package is over 4,500 lines long.

To provide the best of both worlds , GNU egrep Version 2.4.2 utilizes two fully functional engines (about 8,900 lines of code), and Tcl's hybrid DFA/NFA engine (see the sidebar on the facing page) is about 9,500 lines of code.

Some implementations are simple, but that doesn't necessarily mean they are short on features. I once wanted to use regular expressions for some text processing in Pascal. I hadn't used Pascal since college, but it still didn't take long to write a simple NFA regex engine. It didn't have a lot of bells and whistles, and wasn't built for maximum speed, but the flavor was relatively full-featured and was quite useful.