HTML parsing or site grabber, how to extract the necessary data from the page

20.06.18 IT / Html 11907

Parsing site pages laid out in HTML can be easily implemented in almost any programming language. Naturally, this can be a very necessary task, because today there are many different sources, it remains only to take data from them and use it for your needs.

What is parsing ? Parsing is the process of getting and extracting the data you want from any existing data source. What is a parser or site grabber ? It is a tool for extracting the data you need from any source that contains information in a specific structure.

For example, you need to collect all hotels and display them on one site in order to compare them and offer the user the best option. To do this, the popular site aggregators today use parsers to get data from different sites and then display a list of offers on their resource.

Thanks to the ability to parse, you can get almost any data from a document, especially if such a document has a specific structure. HTML page is just such an example. This data source is well structured, which means it's easy to retrieve the data you want. This is extremely in demand at the present time, because there are a very large number of sites, you can easily parse any resource without much effort. But it is also important not to violate the rights to copyright content and other rules, to use HTML parsers only for lawful purposes.

HTML document is built from tags that carry semantic meaning, which means that parsing can be performed on any such tag. But parsing is not necessary only in this way, you can write a regular expression in your parser code, then load the page and go through all its contents - select only what matches the given regular expression. This way you can extract anything you want.

How to develop an HTML parser and how does it work? In short:

• any programming language convenient for you is chosen and a parser program is written,
• the source code of this program is being compiled, which should be able to make requests to sites and receive their HTML content,
• an important part of such code, regular expressions or the rules by which the data will be retrieved depends on what data needs to be retrieved
• the entire content of the page is run and only those that match the created rules are selected, the rest is cut off,
• the results are saved to the location you want.

As you can see, everything is simple. For example, you need get email addresses < / strong> from the site page, then it is enough to compose a regular expression and apply the appropriate function - all unnecessary will be cut off, only the email address will remain, if it is in the HTML code of the page. Regular expressions for this and other cases can be found online if you can't compose them yourself.