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I am looking to create a system that extracts blocks of code from C++ files. For example, if I wanted to extract every while loop, I would look for a pattern that begins with while and ends with }. The problem with that specific example is that while loops may contain other scope blocks, so I'd need to:
Find the string while - regex can easily do this
match braces starting with the open brace after the while and ending with its matching brace
Also match while loops that contain a single line and no braces
Handle as many special cases as possible, such as while loops declared in comments etc, as per #Cid's suggestion
I can do this with a parser and a lot of code, but I was wondering if anything existed that perhaps extends regex to this sort of document level query?
There are parser libraries and tools, even free open-source ones. Clang has one, for example. So does GCC. There are others.
It's a lot of code because C++ is hard to parse. But if someone else gas written the code and it works, that's nit a problem. The usual difficulty with using these products is finding good documentation, but you can always try asking specific questions here
But just doing a lexical analysis of C++ is less difficult, and would be sufficient for crude analysis of program structure if you don't care that it will fail on corner cases. If you start with preprocessed code (or make the dubious assumption that preprocessing doesn't change the program structure) and don't worry about identifying template brackets (in particular, distinguishing between the right shift operator and two consecutive close angle brackets), you should be able to build a lexical analyser with a reasonably short scanner generator specification.
That might be sufficient for crude analysis of program structure if you don't care that it will fail on corner cases.
I have not gotten into the field of formal languages in computer science yet, so maybe my question is silly. I am writing a simple NMEA parser in C++, and I have to choose:
My first idea was to build a simple finite state machine manually, but then I thought that maybe I could do it with less work, even more efficiently. I used regular expressions before, but I think the NMEA regular expression is very long and should take "long time" to match it.
Then I thought about using a parser generator. I think all use the same method: they generate a FSA. But I don't know which is more efficient. When do you normally use parser generators instead of regexes (I think you could write regex in parser generator)?
Please explain the differences, I'm interested in both theory and experience.
Well, a simple rule of thumb is: If the grammar of the data you are trying to parse is regular, use regular expressions. If it is not, regular expressions may still work (as most regex engines also support non-regular grammars), but it might well be painful (complicated / bad performance).
Another aspect is what you are trying to do with the parsed data. If you are only interested in one field, a regex is probably easier to read. If you need to read deeply nested structures, a parser is likely to be more maintainable.
Regex is a parser-generator.
From wikipedia:
Regular expressions (abbreviated as regex or regexp, with plural forms regexes, regexps, or regexen) are written in a formal language that can be interpreted by a regular expression processor, a program that either serves as a parser generator or examines text and identifies parts that match the provided specification.
If you're going over a list that only needs to be gone over once, then save the list to a file and read it from there. If you're checking things that are different every time, use regex and store the results in an array or something.
It's much faster than you would assume it to be. I've seen expressions bigger than this post.
Adding that you can nest as much as you'd like, in whatever language you decide to code it in. You could even do it in sections, for maximum re-usability.
As Sneakyness points out, you can have a large and complicated regular expression that is surprisingly powerful. I've seen some examples of this, but none were maintainable by mere mortals. Even using Expresso only helped so much; it was still difficult to understand and risky to modify. So unless you're a savant with a fixation on Grep, I would not recommend this direction.
Instead, consider focusing on the grammar and letting a compiler compiler do the heavy lifting for you.
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Someone I know has been telling me that RegEx should be avoided, as it is heavyweight or involves heavy processing. Is this true? This made a clap in my ears, ringing my eardrums up until now.
I don't know why he told me that. Could it have been from experience or merely 3rd-hand information (you know what I mean...)?
So, stated plainly, why should I avoid regular expressions?
If you can easily do the same thing with common string operations, then you should avoid using a regular expression.
In most situations regular expressions are used where the same operation would require a substantial amount of common string operations, then there is of course no point in avoiding regular expressions.
Don't avoid them. They're an excellent tool, and when used appropriately can save you a lot of time and effort. Moreover, a good implementation used carefully should not be particularly CPU-intensive.
Overhyped? No. They're extremely powerful and flexible.
Overused? Absolutely. Particularly when it comes to parsing HTML (which frequently comes up here).
This is another of those "right tool for the job" scenarios. Some go too far and try to use it for everything.
You are right though in that you can do many things with substring and/or split. You will often reach a point with those where what you're doing will get so complicated that you have to change method or you just end up writing too much fragile code. Regexes are (relatively) easy to expand.
But hand written code will nearly always be faster. A good example of this is Putting char into a java string for each N characters. The regex solution is terser but has some issues that a hand written loop doesn't and is much slower.
You can substitute "regex" in your question with pretty much any technology and you'll find people who poorly understand the technology or too lazy to learn the technology making such claims.
There is nothing heavy-weight about regular expressions. The most common way that programmers get themselves into trouble using regular expressions is that they try to do too much with a single regular expression. If you use regular expressions for what they're intended (simple pattern matching), you'll be hard-pressed to write procedural code that's more efficient than the equivalent regular expression. Given decent proficiency with regular expressions, the regular expression takes much less time to write, is easier to read, and can be pasted into tools such as RegexBuddy for visualization.
As a basic parser or validator, use a regular expression unless the parsing or validation code you would otherwise write would be easier to read.
For complex parsers (i.e. recursive descent parsers) use regex only to validate lexical elements, not to find them.
The bottom line is, the best regex engines are well tuned for validation work, and in some cases may be more efficient than the code you yourself could write, and in others your code would perform better. Write your code using handwritten state machines or regex as you see fit, but change from regex to handwritten code if performance tests show you that regex is significantly inefficient.
"When you have a hammer, everything looks like a nail."
Regular expressions are a very useful tool; but I agree that they're not necessary for every single place they're used. One positive factor to them is that because they tend to be complex and very heavily used where they are, the algorithms to apply regular expressions tend to be rather well optimized. That said, the overhead involved in learning the regular expressions can be... high. Very high.
Are regular expressions the best tool to use for every applicable situation? Probably not, but on the other hand, if you work with string validation and search all the time, you probably use regular expressions a lot; and once you do, you already have the knowledge necessary to use the tool probably more efficiently and quickly than any other tool. But if you don't have that experience, learning it is effectively a drag on your productivity for that implementation. So I think it depends on the amount of time you're willing to put into learning a new paradigm, and the level of rush involved in your project. Overall, I think regular expressions are very worth learning, but at the same time, that learning process can, frankly, suck.
I think that if you learn programming in language that speaks regular expressions natively you'll gravitate toward them because they just solve so many problems. IE, you may never learn to use split because regexec() can solve a wider set of problems and once you get used to it, why look anywhere else?
On the other hand, I bet C and C++ programmers will for the most part look at other options first, since it's not built into the language.
You know, given the fact that I'm what many people call "young", I've heard too much criticism about RegEx. You know, "he had a problem and tried to use regex, now he has two problems".
Seriously, I don't get it. It is a tool like any other. If you need a simple website with some text, you don't need PHP/ASP.NET/STG44. Still, no discussion on whether any of those should be avoided. How odd.
In my experience, RegEx is probably the most useful tool I've ever encountered as a developer. It's the most useful tool when it comes to #1 security issue: parsing user input. I has saved me hours if not days of coding and creating potentially buggy (read: crappy) code.
With modern CPUs, I don't see what's the performance issue here. I'm quite willing to sacrifice some cycles for some quality and security. (It's not always the case, though, but I think those cases are rare.)
Still, RegEx is very powerful. With great power, comes great responsibility. It doesn't mean you'll use it whenever you can. Only where it's power is worth using.
As someone mentioned above, HTML parsing with RegEx is like a Russian roulette with a fully loaded gun. Don't overdo anything, RegEx included.
You should also avoid floating-point numbers at all cost. That is when you're programming in an embedded-environment.
Seriously: if you're in normal software-development you should actually use regex if you need to do something that can't be achieved with simpler string-operations. I'd say that any normal programmer won't be able to implement something that's best done using regexps in a way that is faster than the correspondig regular expression. Once compiled, a regular expression works as a state-maschine that is optimized to near perfection.
Overhyped? No
Under-Utilized Properly? Yes
If more people knew how to use a decent parser generator, there would be fewer people using regular expressions.
In my belief, they are overused by people quite a bit (I've had this discussion a number of times on SO).
But they are a very useful construct because they deliver a lot of expressive power in a very small piece of code.
You only have to look at an example such as a Western Australian car registration number. The RE would be
re.match("[1-9] [A-Z]{3} [0-9]{3}")
whilst the code to check this would be substantially longer, in either a simple 9-if-statement or slightly better looping version.
I hardly ever use complex REs in my code because:
I know how the RE engines work and I can use domain knowledge to code up faster solutions (that 9-if variant would almost certainly be faster than a one-shot RE compile/execute cycle); and
I find code more readable if it's broken up logically and commented. This isn't easy with most REs (although I have seen one that allows inline comments).
I have seen people suggest the use of REs for extracting a fixed-size substring at a fixed location. Why these people don't just use substring() is beyond me. My personal thought is that they're just trying to show how clever they are (but it rarely works).
Don't avoid it, but ask youself if they're the best tool for the task you have to solve. Maybe sometimes regex are difficult to use or debug, but they're really usefull in some situations. The question is to use the apropiate tool for each task, and usually this is not obvious.
There is a very good reason to use regular expressions in scripting languages (such as Ruby, Python, Perl, JavaScript and Lua): parsing a string with carefully optimized regular expression executes faster than the equivalent custom while loop which scans the string character-by-character. For compiled languages (such as C and C++, and also C# and Java most of the time) usually the opposite is true: the custom while loop executes faster.
One more reason why regular expressions are so popular: they express the programmer's intention in an extremely compact way: a single-line regexp can do as much as a 10- or 20-line while loop.
Overhyped? No, if you have ever taken a Parsing or Compiler course, you would understand that this is like saying addition and multiplication is overhyped for math problems.
It is a system for solving parsing problems.
some problems are simpler and don't require regular expressions, some are harder and require better tools.
I've seen so many people argue about whether a given regex is correct or not that I'm starting to think the best way to write one is to ask how to do it on StackOverflow and then let the regex gurus fight it out.
I think they're especially useful in JavaScript. JavaScript is transmitted (so should be small) and interpreted from text (although this is changing in the new browsers with V8 and JIT compilation), so a good internal regex engine has a chance to be faster than an algorithm.
I'd say if there is a clear and easy way to do it with string operations, use the string operations. But if you can do a nice regex instead of writing your own state machine interpreter, use the regex.
Regular Expressions are one of the most useful things programmers can learn, they allow to speed up and minimize your code if you know how to handle them.
Regular expressions are often easier to understand than the non-regex equivalent, especially in a language with native regular expressions, especially in a code section where other things that need to be done with regexes are present.
That doesn't meant they're not overused. The only time string.match(/\?/) is better than string.contains('?') is if it's significantly more readable with the surrounding code, or if you know that .contains is implemented with regexes anyway
I often use regex in my IDE to quick fix code. Try to do the following without regex.
glVector3f( -1.0f, 1.0f, 1.0f ); -> glVector3f( center.x - 1.0f, center.y + 1.0f, center.z + 1.0f );
Without regex, it's a pain, but WITH regex...
s/glVector3f\((.*?),(.*?),(.*?)\)/glVector3f(point.x+$1,point.y+$2,point.z+$3)/g
Awesome.
I'd agree that regular expressions are sometimes used inappropriately. Certainly for very simple cases like what you're describing, but also for cases where a more powerful parser is needed.
One consideration is that sometimes you have a condition that needs to do something simple like test for presence of a question mark character. But it's often true that the condition becomes more complex. For example, to find a question mark character that isn't preceded by a space or beginning-of-line, and isn't followed by an alphanumeric character. Or the character may be either a question mark or the Spanish "¿" (which may appear at the start of a word). You get the idea.
If conditions are expected to evolve into something that's less simple to do with a plain call to String.contains("?"), then it could be easier to code it using a very simple regular expression from the start.
It comes down to the right tool for the job.
I usually hear two arguments against regular expressions:
1) They're computationally inefficient, and
2) They're hard to understand.
Honestly, I can't understand how either are legitimate claims.
1) This may be true in an academic sense. A complex expression can double back on itself may times over. Does it really matter though? How many millions of computations a second can a server processor do these days? I've dealt with some crazy expressions, and I've never seen a regexp be the bottle neck. By far it's DB interaction, followed by bandwidth.
2) Hard for about a week. The most complicated regexp is no more complex than HTML - it's just a familiarity problem. If you needed HTML once every 3 months, would you get it 100% each time? Work with them on a daily basis and they're just as clear as any other language syntax.
I write validation software. REGEXP's are second nature. Every fifth line of code has a regexp, and for the life of me I can't understand why people make a big deal about them. I've never seen a regexp slow down processing, and I've seen even the most dull 'programmers' pick up the syntax.
Regexp's are powerful, efficient, and useful. Why avoid them?
I wouldn't say avoid them entirely, as they are QUITE handy at times. However, it is important to realize the fundamental mechanisms underneath. Depending on your implementation, you could have up to exponential run-time for a search, but since searches are usually bounded by some constant number of backtraces, you can end up with the slowest linear run-time you ever saw.
If you want the best answer, you'll have to examine your particular implementation as well as the data you intend to search on.
From memory, wikipedia has a decent article on regular expressions and the underlying algorithms.
Are there any best practices that I should follow while writing a parser?
The received wisdom is to use parser generators + grammars and it seems like good advice, because you are using a rigorous tool and presumably reducing effort and potential for bugs in doing so.
To use a parser generator the grammar has to be context free. If you are designing the languauge to be parsed then you can control this. If you are not sure then it could cost you a lot of effort if you start down the grammar route. Even if it is context free in practice, unless the grammar is enormous, it can be simpler to hand code a recursive decent parser.
Being context free does not only make the parser generator possible, but it also makes hand coded parsers a lot simpler. What you end up with is one (or two) functions per phrase. Which is if you organise and name the code cleanly is not much harder to see than a grammar (if your IDE can show you call hierachies then you can pretty much see what the grammar is).
The advantages:-
Simpler build
Better performance
Better control of output
Can cope with small deviations, e.g. work with a grammar that is not 100% context free
I am not saying grammars are always unsuitable, but often the benefits are minimal and are often out weighed by the costs and risks.
(I believe the arguments for them are speciously appealing and that there is a general bias for them as it is a way of signaling that one is more computer-science literate.)
Few pieces of advice:
Know your grammar - write it down in a suitable form
Choose the right tool. Do it from within C++ with Spirit2x, or choose external parser tools like antlr, yacc, or whatever suits you
Do you need a parser? Maybe regexp will suffice? Or maybe hack a perl script to do the trick? Writing complex parsers take time.
Don't overuse regular expressions - while they have their place, they simply don't have the power to handle any kind of real parsing. You can push them, but you're eventually going to hit a wall or end up with an unmaintainable mess. You're better off finding a parser generator that can handle a larger language set. If you really don't want to get into tools, you can look at recursive descent parsers - it's a really simple pattern for hand-writing a small parser. They aren't as flexible or as powerful as the big parser generators, but they have a much shorter learning curve.
Unless you have very tight performance requirements, try and keep your layers separate - the lexer reads in individual tokens, the parser arranges those into a tree, and then semantic analysis checks over everything and links up references, and then a final phase to output whatever is being produced. Keeping the different parts of logic separate will make things easier to maintain later.
Read most of the Dragon book first.
Parsers are not complicated if you know how to build them, but they are NOT the type of thing that if you put in enough time, you'll eventually get there. It's way better to build on the existing knowledge base. (Otherwise expect to write it and throw it away a few dozen times).
Yep. Try to generate it, not write. Consider using yacc, ANTLR, Flex/Bison, Coco/R, GOLD Parser generator, etc. Resort to manually writing a parser only if none of existing parser generators fit your needs.
Choose the right kind of parser, sometimes a Recursive Descendant will be enough, sometimes you should use an LR parser (also, there are many types of LR parsers).
If you have a complex grammar, build an Abstract Syntax Tree.
Try to identify very well what goes into the lexer, what is part of the syntax and what is a matter of semantics.
Try to make the parser the least coupled to the lexer implementation as possible.
Provide a good interface to the user so he is agnostic of the parser implementation.
First, don't try to apply the same techniques to parsing everything. There are numerous possible use cases, from something like IP addresses (a bit of ad hoc code) to C++ programs (which need an industrial-strength parser with feedback from the symbol table), and from user input (which needs to be processed very fast) to compilers (which normally can afford to spend a little time parsing). You might want to specify what you're doing if you want useful answers.
Second, have a grammar in mind to parse with. The more complicated it is, the more formal the specification needs to be. Try to err on the side of being too formal.
Third, well, that depends on what you're doing.
It seems that the choice to use string parsing vs. regular expressions comes up on a regular basis for me anytime a situation arises that I need part of a string, information about said string, etc.
The reason that this comes up is that we're evaluating a soap header's action, after it has been parsed into something manageable via the OperationContext object for WCF and then making decisions on that. Right now, the simple solution seems to be basic substring'ing to keep the implementation simple, but part of me wonders if RegEx would be better or more robust. The other part of me wonders if it'd be like using a shotgun to kill a fly in our particular scenario.
So I have to ask, what's the typical threshold that people use when trying to decide to use RegEx over typical string parsing. Note that I'm not very strong in Regular Expressions, and because of this, I try to shy away unless it's absolutely vital to avoid introducing more complication than I need.
If you couldn't tell by my choice of abbreviations, this is in .NET land (C#), but I believe that doesn't have much bearing on the question.
EDIT: It seems as per my typical Raybell charm, I've been too wordy or misleading in my question. I want to apologize. I was giving some background to help give clues as to what I was doing, not mislead people.
I'm basically looking for a guideline as to when to use substring, and variations thereof, over Regular Expressions and vice versa. And while some of the answers may have missed this (and again, my fault), I've genuinely appreciated them and up-voted as accordingly.
My main guideline is to use regular expressions for throwaway code, and for user-input validation. Or when I'm trying to find a specific pattern within a big glob of text. For most other purposes, I'll write a grammar and implement a simple parser.
One important guideline (that's really hard to sidestep, though I see people try all the time) is to always use a parser in cases where the target language's grammar is recursive.
For example, consider a tiny "expression language" for evaluating parenthetized arithmetic expressions. Examples of "programs" in this language would look like this:
1 + 2
5 * (10 - 6)
((1 + 1) / (2 + 2)) / 3
A grammar is easy to write, and looks something like this:
DIGIT := ["0"-"9"]
NUMBER := (DIGIT)+
OPERATOR := ("+" | "-" | "*" | "/" )
EXPRESSION := (NUMBER | GROUP) (OPERATOR EXPRESSION)?
GROUP := "(" EXPRESSION ")"
With that grammar, you can build a recursive descent parser in a jiffy.
An equivalent regular expression is REALLY hard to write, because regular expressions don't usually have very good support for recursion.
Another good example is JSON ingestion. I've seen people try to consume JSON with regular expressions, and it's INSANE. JSON objects are recursive, so they're just begging for regular grammars and recursive descent parsers.
Hmmmmmmm... Looking at other people's responses, I think I may have answered the wrong question.
I interpreted it as "when should use use a simple regex, rather than a full-blown parser?" whereas most people seem to have interpreted the question as "when should you roll your own clumsy ad-hoc character-by-character validation scheme, rather than using a regular expression?"
Given that interpretation, my answer is: never.
Okay.... one more edit.
I'll be a little more forgiving of the roll-your-own scheme. Just... don't call it "parsing" :o)
I think a good rule of thumb is that you should only use string-matching primitives if you can implement ALL of your logic using a single predicate. Like this:
if (str.equals("DooWahDiddy")) // No problemo.
if (str.contains("destroy the earth")) // Okay.
if (str.indexOf(";") < str.length / 2) // Not bad.
Once your conditions contain multiple predicates, then you've started inventing your own ad hoc string validation language, and you should probably just man up and study some regular expressions.
if (str.startsWith("I") && str.endsWith("Widget") &&
(!str.contains("Monkey") || !str.contains("Pox"))) // Madness.
Regular expressions really aren't that hard to learn. Compared to a huuuuge full-featured language like C# with dozens of keywords, primitive types, and operators, and a standard library with thousands of classes, regular expressions are absolutely dirt simple. Most regex implementations support about a dozen or so operations (give or take).
Here's a great reference:
http://www.regular-expressions.info/
PS: As a bonus, if you ever do want to learn about writing your own parsers (with lex/yacc, ANTLR, JavaCC, or other similar tools), learning regular expressions is a great preparation, because parser-generator tools use many of the same principles.
The regex can be
easier to understand
express more clearly the intent
much shorter
easier to change/adapt
In some situations all of those advantages would be achieved by using a regex, in others only some are achieved (the regex is not really easy to understand for example) and in yet other situations the regex is harder to understand, obfuscates the intent, longer and hard to change.
The more of those (and possibly other) advantages I gain from the regex, the more likely I am to use them.
Possible rule of thumb: if understanding the regex would take minutes for someone who is somewhat familiar with regular expressions, then you don't want to use it (unless the "normal" code is even more convoluted ;-).
Hm ... still no simple rule-of-thumb, sorry.
[W]e're evaluating a soap header's
action and making decisions on that
Never use regular expressions or basic string parsing to process XML. Every language in common usage right now has perfectly good XML support. XML is a deceptively complex standard and it's unlikely your code will be correct in the sense that it will properly parse all well-formed XML input, and even it if does, you're wasting your time because (as just mentioned) every language in common usage has XML support. It is unprofessional to use regular expressions to parse XML.
To answer your question, in general the usage of regular expressions should be minimized as they're not very readable. Oftentimes you can combine string parsing and regular expressions (perhaps in a loop) to create a much simpler solution than regular expressions alone.
I would agree with what benjismith said, but want to elaborate just a bit. For very simple syntaxes, basic string parsing can work well, but so can regexes. I wouldn't call them overkill. If it works, it works - go with what you find simplest. And for moderate to intermediate string parsing, a regex is usually the way to go.
As soon as you start finding yourself needing to define a grammar however, i.e. complex string parsing, get back to using some sort of finite state machine or the likes as quickly as you can. Regexes simply don't scale well, to use the term loosely. They get complex, hard to interpret, and even incapable.
I've seen at least one project where the use of regexes kept growing and growing and soon they had trouble inserting new functionality. When it finally came time to do a new major release, they dumped all the regexes and went the route of a grammar parser.
When your required transformation isn't basic -- but is still conceptually simple.
no reason to pull out Regex if you're doing a straight string replacement, for example... its easier to just use the string.Replace
on the other hand, a complex rule with many conditionals or special cases that would take more than 50 characters of regex can be a nightmare to maintain later on if you don't explicitly write it out
I would always use a regex unless it's something very simple such as splitting a comma-separated string. If I think there's a chance the strings might one day get more complicated, I'll probably start with a regex.
I don't subscribe to the view that regexes are hard or complicated. It's one tool that every developer should learn and learn well. They have a myriad of uses, and once learned, this is exactly the sort of thing you never have to worry about ever again.
Regexes are rarely overkill - if the match is simple, so is the regex.
I would think the easiest way to know when to use regular expressions and when not to, is when your string search requires an IF/THEN statement or anything resembling this or that logic, then you need something better than a simple string comparison which is where regex shines.