3 ms·

It's possible to a certain degree, but it's just way way harder then you'd do if you used a proper parser. For example, let's say I want to get all the recent q

by inglor 11y ago

It's possible to a certain degree, but it's just way way harder then you'd do if you used a proper parser. For example, let's say I want to get all the recent question links off http://stackoverflow.com/questions/new http://stackoverflow.com/questions/new , With RegExp, I can write a clever thing that parses `<a href="(.?*)" class="question-link"` and _hope_ that the order never changes and the class never comes before the href and that they don't change that in a design or in other pages and that the href does not contain `class="question-link` inside it and to a degree that's valid. The thing is - the alternative is to write a query selector - which is another more suitable domain specific language for making selections - only on the DOM instead of text, I'd just write `$(".question-link").get().map(x => x.href)` to get the hrefs and I know it's __always perfectly safe__. Now that example is trivial, if I only want links where the questions are tagged with C#, I get a much harder problem with Regex, but with query selectors it's still mostly trivial. So, it's not that it's particularly hard to use regular expressions to solve it, it's just a lot harder than the alternative which is super simple and obviously correct.

joosters 11y ago
Your method relies on the class names not being renamed (e.g. I see "question-hyperlink" on /questions, not "question-link"). I'd skip the class entirely and match on the URL, since I doubt stackoverflow want their URLs to break: /<a[^>]*href="(\/questions\/\d+[^"]+)"/i But... we can go back and forwards posting examples and find fault in any regex that I post or any selector that you post. It's missing the point. Both methods are at the mercy of web page redesigns. Both methods can be made more robust against certain changes, but cannot survive other changes. You are trying to say that regexes won't work. I am saying that both methods work.
ifdefdebug 11y ago
Look, you have essentially two options here: 1) add a full featured DOM parser to your program - do that if you have to understand the DOM; 2) write regex - do that if you don't care about the DOM, if you know how the server formats the data you need and if the server formats the data you need in a reproducible way. > (...) and that they don't change that in a design or in other pages (...) Well if they change the design of the pages then you will have to rewrite your regex accordingly in order to find the data you need. But if that happens, odds are high that your program, which uses a full featured DOM parser, will have to be rewritten as well in order to handle the modified output of the DOM parser...
- kragen 11y ago
  in fact, in practice, i find that scrapers using regexp matchers are more robust against the kinds of template changes that people make than scrapers based on the html tree structure.

by inglor 11y ago

joosters 11y ago

Your method relies on the class names not being renamed (e.g. I see "question-hyperlink" on /questions, not "question-link"). I'd skip the class entirely and match on the URL, since I doubt stackoverflow want their URLs to break: /<a[^>]*href="(\/questions\/\d+[^"]+)"/i But... we can go back and forwards posting examples and find fault in any regex that I post or any selector that you post. It's missing the point. Both methods are at the mercy of web page redesigns. Both methods can be made more robust against certain changes, but cannot survive other changes. You are trying to say that regexes won't work. I am saying that both methods work.

ifdefdebug 11y ago

Look, you have essentially two options here: 1) add a full featured DOM parser to your program - do that if you have to understand the DOM; 2) write regex - do that if you don't care about the DOM, if you know how the server formats the data you need and if the server formats the data you need in a reproducible way. > (...) and that they don't change that in a design or in other pages (...) Well if they change the design of the pages then you will have to rewrite your regex accordingly in order to find the data you need. But if that happens, odds are high that your program, which uses a full featured DOM parser, will have to be rewritten as well in order to handle the modified output of the DOM parser...

kragen 11y ago
in fact, in practice, i find that scrapers using regexp matchers are more robust against the kinds of template changes that people make than scrapers based on the html tree structure.