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Is there a regular expression to detect a valid regular expression?

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Re: Is there a regular expression to detect a valid regular expression?

#141

Post by mannykannot »

ukj wrote: post #115
mannykannot wrote: post #108

Earlier quoted context omitted.

Go ahead - show me wrong.

I have already done that, what I am unsure of is how to go about convincing you. With compilers - I just fix the bug myself. With humans, I have to convince the human to self-correct. Common problem in distributed systems that - leader election.

It is a general rule that those who avoid answering a question do not have an answer, and this is no exception. Here, You completely misunderstand Chomsky’s hierarchy: By your inverted-hierarchy argument, the simplest regular language would be complex enough that incompleteness would be an issue in its validation.

Re: Is there a regular expression to detect a valid regular expression?

#142

Post by sixstringtheory »

nightfly wrote: post #140
jessaustin wrote: post #95

Earlier quoted context omitted.

Just considering the language of balanced parentheses, the only memory needed is a single integer to track depth. If we see a "(", increment. If we see a ")", decrement. If we never go negative and end up with zero, that's a balanced parentheses expression. (To be clear, the parser I've described is not a regex parser.) Adding the rest of the regex machinery makes this a bit more complex, but in general, if the compu…

)))((()(()

Good edge case. I’d say to immediately return an ‘unbalanced’ result once the count goes negative.

Re: Is there a regular expression to detect a valid regular expression?

#144

Post by NieDzejkob »

thereare5lights wrote: post #138
fortran77 wrote: post #132

We see good examples of "the problem with StackOverflow" here. The second highest-rated answer is "Evaluate it in a try..catch or whatever your language provides." and it's justified because "Surely the real question is 'how do I validate a regular expression'." This is a fascinating computer science question and I'm pretty sure the questioner wasn't asking "how do I validate a regular expression" because he would ha…

> I'm pretty sure the questioner wasn't asking "how do I validate a regular expression" because he would have asked that. In my experience, people sometimes ask for how to solve the more immediate detail they’re working on rather than the broader problem.

See also: the XY problem [0][1].

[0]: http://xyproblem.info/

[1]: https://en.wikipedia.org/wiki/XY_problem

Re: Is there a regular expression to detect a valid regular expression?

#145

Post by avar »

fortran77 wrote: post #132

We see good examples of "the problem with StackOverflow" here. The second highest-rated answer is "Evaluate it in a try..catch or whatever your language provides." and it's justified because "Surely the real question is 'how do I validate a regular expression'." This is a fascinating computer science question and I'm pretty sure the questioner wasn't asking "how do I validate a regular expression" because he would ha…

It really doesn't matter what the questioner wanted. The main utility of sites like Stack Overflow is not to give this one person the answer to their specific problem, but to be a useful landing page to people searching for things related to the question later.

A lot of those people are novices when it comes to regexes, and it's useful to make it clear that there's some things that are better solved with other tools, even if you can abuse extended regexes syntaxes to solve them.

Re: Is there a regular expression to detect a valid regular expression?

#146

Post by LoSboccacc »

hk__2 wrote: post #129
LoSboccacc wrote: post #105

Earlier quoted context omitted.

it's sad that today the stack overflow majority shares your mindset and an answer like that would drown in downvotes or killed by moderation. some people like to act super serious all the time like they're playing a sitcom version of what they think adulthood is in a quest to be the most boring person on earth like if that's the goal of human interaction

You can be funny and helpful at the same time.

which is exactly what that so answer was

Re: Is there a regular expression to detect a valid regular expression?

#147

Post by tom_mellior »

tzs wrote: post #124
gamache wrote: post #2

I believe Zalgo has the answer to this, via an equivalent question. https://stackoverflow.com/questions/1732348/regex-match-open...

From that answer: "You can't parse [X]HTML with regex". I've always wondered how that came about. How come very early in the development of the web no one important enough for people to pay attention to them said, "Hey...wait a second. If this thing becomes popular, people are going to really want to processes web documents with their usual text file processing tools and techniques. We really outta make this thing re…

The problem isn't with the surface syntax of HTML. Anything else, like S-expressions or JSON or whatever would have the same problem. The problem is arbitrary nesting of elements. No language with nesting can be parsed with regex (modulo various copouts mentioned elsethread). But such nesting is useful and necessary for a document language like HTML. This goal is important; "we must be able to use awk" isn't.

Re: Is there a regular expression to detect a valid regular expression?

#148

Post by mannykannot »

avar wrote: post #145
fortran77 wrote: post #132

We see good examples of "the problem with StackOverflow" here. The second highest-rated answer is "Evaluate it in a try..catch or whatever your language provides." and it's justified because "Surely the real question is 'how do I validate a regular expression'." This is a fascinating computer science question and I'm pretty sure the questioner wasn't asking "how do I validate a regular expression" because he would ha…

It really doesn't matter what the questioner wanted. The main utility of sites like Stack Overflow is not to give this one person the answer to their specific problem, but to be a useful landing page to people searching for things related to the question later. A lot of those people are novices when it comes to regexes, and it's useful to make it clear that there's some things that are better solved with other tools,…

The answer in this case does not even satisfy that standard.

Re: Is there a regular expression to detect a valid regular expression?

#149

Post by corobo »

thereare5lights wrote: post #138
fortran77 wrote: post #132

We see good examples of "the problem with StackOverflow" here. The second highest-rated answer is "Evaluate it in a try..catch or whatever your language provides." and it's justified because "Surely the real question is 'how do I validate a regular expression'." This is a fascinating computer science question and I'm pretty sure the questioner wasn't asking "how do I validate a regular expression" because he would ha…

> I'm pretty sure the questioner wasn't asking "how do I validate a regular expression" because he would have asked that. In my experience, people sometimes ask for how to solve the more immediate detail they’re working on rather than the broader problem.

This came up recently. Going to start calling it the XYZ problem

https://news.ycombinator.com/item?id=20861806

Edit: page refreshed and sure enough the sibling comment calls it out by name

Edit2: Called it the XYZ problem: https://cohan.io/the-xyz-problem/

Re: Is there a regular expression to detect a valid regular expression?

#150

Post by mholt »

fortran77 wrote: post #132

We see good examples of "the problem with StackOverflow" here. The second highest-rated answer is "Evaluate it in a try..catch or whatever your language provides." and it's justified because "Surely the real question is 'how do I validate a regular expression'." This is a fascinating computer science question and I'm pretty sure the questioner wasn't asking "how do I validate a regular expression" because he would ha…

I also think it's a fascinating question, but if the asker wanted a theoretical answer more than a practical one, it should have been asked on the CS or CS theory stack exchange: https://cs.stackexchange.com/ & https://cstheory.stackexchange.com/
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