Tokenizing input with the regular expression library
When parsing or transforming strings in complex ways or breaking them into chunks, regular expressions are a great help. In many programming languages, they are already built in because they are so useful and handy.
If you do not know regular expressions yet, have a look at the Wikipedia article about them, for example. They will surely extend your horizon, as it is easy to see how useful they are when parsing any kind of text. Regular expressions can, for example, test whether an e-mail address string or an IP address string is valid, find and extract substrings out of large strings, which follow a complex pattern, and so on.
In this recipe, we will extract all the links out of an HTML file and list them for the user. The code will be amazingly short because we have regular expression support built in the C++ STL since C++11.
How to do it...
We are going to define a regular expression that detects links, and we apply it to an HTML file in...
