I'm wondering why there have to be so many regular expression dialects. Why does it seem like so many languages, rather then reusing a tried and true dialect, seem bent on writing their own.
Like these.
I mean, I understand that some of these do have very different backends. But shouldn't that be abstracted from the programmer?
I'm more referring to the odd but small differences, like where parentheses have to be escaped in one language, but are literals in another. Or where meta-characters mean somewhat different things.
Is there any particular reason we can't have some sort of universal dialect for regular expressions? I would think it would make things much easier for programmers who have to work in multiple languages.
Because regular expressions only have three operations:
Concatenation
Union |
Kleene closure *
Everything else is an extension or syntactic sugar, and so has no source for standardization. Things like capturing groups, backreferences, character classes, cardinality operations, etc are all additions to the original definition of regular expressions.
Some of these extensions make "regular expressions" no longer regular at all. They are able to decide non-regular languages because of these extras, but we still call them regular expressions regardless.
As people add more extensions, they will often try to use other, common variations of regular expressions. That's why nearly every dialect uses X+ to mean "one or many Xs", which itself is just a shortcut for writing XX*.
But when new features get added, there's no basis for standardization, so someone has to make something up. If more than one group of designers come up with similar ideas at around the same time, they'll have different dialects.
For the same reason we have so many languages. Some people will be trying to improve their tools and at the same time others will be resistant to change. C/C++/Java/C# anyone?
The "I made it better" syndrome of programming produces all these things. It's the same with standards. People try to make the next "best" standard to replace all the others and it just becomes something else we all have to learn/design for.
I think a good part of this is the question of who would be responsible for setting and maintaining the standard syntax and ensuring compatibility accross differing environments.
Also, if a regex must itself be parsed inside an interpreter/compiler with its own unique rules regarding string manipulation, then this can cause a need for doing things differently with regard to escapes and literals.
A good strategy is to take time to understand how regex algorithms themselves function at a more abstract level; then implementing any particular syntax becomes much easier. Similar to how each programming language has its own syntax for constructs like conditional statements and loops, but still accomplish the same abstract task.
Related
With the construct of using If-Then-Else Conditionals in regular expressions, I would like to know the possible outcome of trying to manipulate many constructs into a single expression for multiple matches.
Let's take this example below.
foo(bar)?(?(1)baz|quz)
Now being combined with an expression, which matches the previous conditions and then we add on to the previous with the following conditions..
foo(bar)?(?(1)baz|quz)|(?(?=.*baz)bar|foo)
Mainly I am asking should you construct a regular expression in this way, and what would ever be the purpose that you would need to use it in this way?
should you construct a regular expression in this way, and what would
ever be the purpose that you would need to use it in this way?
In this case, and probably most cases, I would say "no".
I often find that conditionals can be rewritten as lookarounds or simplified within alternations.
For instance, it seems to me that the regex you supplied,
foo(bar)?(?(1)baz|quz)|(?(?=.*baz)bar|foo)
could be replaced for greater clarity by
bar(?=.*baz)|foo(?:quz|barbaz)?
which gives us two simple match paths.
But it's been six months since you posted the question. During this time, answering a great many questions on SO, have you ever felt the need for that kind of construction?
I believe the answer to this would be ultimately be specific to the regex library being used or a language's implementation. ("Comparison of regular expression engines", Wikipedia.)
There isn't an official RFC or specification for regular expressions. And the diversity of implementations leads to frustration doing even "simple" expressions--the nuances you're considering are probably implementation-specific instead of regex-specific.
And even beyond your specific question, I think most of the regex-related questions on StackOverflow would be improved if people were more specific about the language being used or the library employed by the application they're using. When troubleshooting my own regular expressions in various applications (text editors for example) it took me awhile before I realized the need to understand the specific library being used.
I know what you're thinking - "oh my god, seriously, not again" - but please bear with me, my question is more than the title. Before we begin, I promise I will never try to parse arbitrary HTML with a regex, or ask anyone else how.
All of the many, many answers here explaining why you cannot do this rely on the formal definition of regular expressions. They parse regular languages, HTML is context-free but not regular, so you can't do it. But I have also heard that many regex implementations in various languages are not strictly regular; they come with extra tricks that break outside the bounds of formal regular expressions.
Since I don't know the details of any particular implementations, such as perl, my questions are:
Which features of regex tools are non-regular? Is it the back references? And in which languages are they found?
Are any of these extra tricks sufficient to parse all context-free languages?
If "no" to #2, then is there a formal category or class of languages that these extra features cover exactly? How can we quickly know whether the problem we are trying to solve is within the power of our not-necessarily-regular expressions?
The answer to your question is that yes, so-called “extended regexes” — which are perhaps more properly called patterns than regular expressions in the formal sense — such as those found in Perl and PCRE are indeed capable of recursive descent parsing of context-free grammars.
This posting’s pair of approaches illustrate not so much theoretical as rather practical limits to applying regexes to X/HTML. The first approach given there, the one labelled naïve, is more like the sort you are apt to find in most programs that make such an attempt. This can be made to work on well-defined, non-generic X/HTML, often with very little effort. That is its best application, just as open-ended X/HTML is its worst.
The second approach, labelled wizardly, uses an actual grammar for parsing. As such, it is fully as powerful as any other grammatical approach. However, it is also far beyond the powers of the overwhelming majority of casual programmers. It also risks re-creating a perfectly fine wheel for negative benefit. I wrote it to show what can be done, but which under virtually no circumstances whatsoever ever should be done. I wanted to show people why they want to use a parser on open-ended X/HTML by showing them how devilishly hard it is to come even close to getting right even using some of the most powerful of pattern-matching facilities currently available.
Many have misread my posting as somehow advocating the opposite of what I am actually saying. Please make no mistake: I’m saying that it is far too complicated to use. It is a proof by counter-example. I had hoped that by showing how to do it with regexes, people would realize why they did not want to go down that road. While all things are possible, not all are expedient.
My personal rule of thumb is that if the required regex is of only the first category, I may well use it, but that if it requires the fully grammatical treatment of the second category, I use someone else’s already-written parser. So even though I can write a parser, I see no reason to do so, and plenty not to.
When carefully crafted for that explicit purpose, patterns can be more resisilient to malformed X/HTML than off-the-shelf parsers tend to be, particularly if you have no real opportunity to hack on said parsers to make them more resilient to the common failure cases that web browsers tend to tolerate but validators do not. However, the grammatical patterns I provide above were designed for only well-formed but reasonably generic HTML (albeit without entity replacement, which is easily enough added). Error recovery in parsers is a separate issue altogether, and by no means a pleasant one.
Patterns, especially the far more commonplace non-grammatical ones most people are used to seeing and using, are much better suited for grabbing up discrete chunks one at a time than they are for producing a full syntactic analysys. In other words, regexes usually work better for lexing than they do for parsing. Without grammatical regexes, you should not try parsing grammars.
But don’t take that too far. I certainly do not mean to imply that you should immediately turn to a full-blown parser just because you want to tackle something that is recursively defined. The easiest and perhaps most commonly seen example of this sort of thing is a pattern to detect nested items, like parentheses. It’s extremely common for me to just plop down something simple like this in my code, and be done with it:
# delete all nested parens
s/\((?:[^()]*+|(?0))*\)//g;
Yes, the extensions in questions are backreferences, and they technically make "regexps" NP-complete, see the Wikipedia paragraph.
I know there is the perl regex that is sort of a minor de facto standard, but why hasn't anyone come up with a universal set of standard symbols, syntax and behaviors?
There is a standard by IEEE associated with the POSIX effort. The real question is "why doesn't everyone follow it"? The answer is probably that it is not quite as complex as PCRE (Perl Compatible Regular Expression) with respect to greedy matching and what not.
Actually, there is a regular expression standard (POSIX), but it's crappy. So people extend their RE engine to fit the needs of their application. PCRE (Perl-compatible regular expressions) is a pseudo-standard for regular expressions that are compatible with Perl's RE engine. This is particularly relevant because you can embed Perl's engine into other applications.
Because making standards is hard. It's nearly impossible to get enough people to agree on anything to make it an official standard, let alone something as complex as regex. Defacto standards are much easier to come by.
Case in point: HTML 5 is not expected to become an official standard until the year 2022. But the draft specification is already available, and major features of the standard will begin appearing in browsers long before the standard is official.
I have researched this and could not find anything concrete. My guess is that it's because regex is so often a tool that works ON tools and therefore it's going to necessarily have platform- and tool- specific extensions.
For example, in Visual Studio, you can use regular expressions to find and replace strings in your source code. They've added stuff like :i to match an identifier. On other platforms in other tools, identifiers may not be an applicable concept. In fact, perhaps other platforms and tools reserve the colon character to escape the expression.
Differences like that make this one particularly hard to standardize.
Perl was first (or danm near close to first), and while it's perl and we all love it, it's old some people felt it needed more polish (i.e. features). This is where new types came in.
They're starting to nomalize, the regex used in .NET is very similar to the regex used in other languages, i think slowly people are starting to unify, but some are used to thier perl ways and dont want to change.
Just a guess: there was never a version popular enough to be considered the canonical standard, and there was no standard implementation. Everyone who came and reimplemented it had their own ideas on how to make it "better".
Because too many people are scared of regular expressions, so they haven't become fully widespread enough for enough sensible people to both think of the idea and be in a position to implement it.
Even if a standards body did form and try to unify the different flavours, too many people would argue stubbornly towards their own approach, whether better or not, because lots of programmers are annoying like that.
While editing this and that in Vim, I often find that its syntax highlighting (for some filetypes) has some defects. I can't remember any examples at the moment, but someone surely will. Usually, it consists of strings badly highlighted in some cases, some things with arithmetic and boolean operators and a few other small things as well.
Now, vim uses regexes for that kinda stuff (its own flavour).
However, I've started to come across editors which, at first glance, have syntax highlighting better taken care of. I've always thought that regexes are the way to go for that kind of stuff.
So I'm wondering, do those editors just have better written regexes, or do they take care of that in some other way ? What ? How is syntax highlighting taken care of when you want it to be "stable" ?
And in your opinion what is the editor that has taken care it the best (in your editor of choice), and how did he do it (language-wise) ?
Edit-1: For example, editors like Emacs, Notepad2, Notepad++, Visual Studio - do you perchance know what mechanism they use for syn. high. ?
The thought that immediately comes to mind for what you'd want to use instead of regexes for syntax highlighting is parsing. Regexes have a lot of advantages, but as we see with vim's highlighting, there are limits. (If you look for threads about using regexes to analyze XML, you'll find extensive material on why regexes can't do what parsers do.)
Since what we want from syntax highlighting is for it to follow the syntactic structure of the language, which regexes can only approximate, you need to perform some level of real parsing to go beyond what regexes can do. A simple recursive descent lexer will probably do great for most languages, I'm thinking.
Some programming languages have a formal definition/specification written in Backus-Naur Form. All*) programming languages can be described in it. All you then need, is some kind of parser for the notation.
*) not verified
For instance, C's BNF definition is "only five pages long".
If you want accurate highlighting one needs real programming not regular expressions. RegExs are rarely the answer fir anything but trivial tasks. To do highlighting in a better way you need to write a simple parser. Parses basically have separate components that each can do something like identify and consume a quoted string or number literal. If said component when looking at it's given cursor can't consume what's underneath it does nothing. From that you can easily parse or highlight fairly simply and easily.
Given something like
static int field = 123;
• The first macher would skip the whitespace before "static". The keyword, literal etc matchers would do nothing because handling whitespace is not their thing.
• The keyword matched when positioned over "static" would consume that. Because "s" is not a digit the literal matched does nothing. The whitespace skipper does nothing as well because "s" is not a whitespace character.
Naturally your loop continues to advance the cursor over the input string until the end is reached. The ordering of your matchers is of course important.
This approach is both flexible in that it handles syntactically incorrect fragments and is also easy to extend and reuse individual matchers to support highlighting of other languages...
I suggest the use of REs for syntax highlighting. If it's not working properly, then your RE isn't powerful or complicated enough :-) This is one of those areas where REs shine.
But given that you couldn't supply any examples of failure (so we can tell you what the problem is) or the names of the editors that do it better (so we can tell you how they do it), there's not a lot more we'll be able to give you in an answer.
I've never had any trouble with Vim with the mainstream languages and I've never had a need to use weird esoteric languages, so it suits my purposes fine.
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.