dart regex matching and get some information from it

Question

For practice, I decided to build something like a Backbone router. The user only needs to give the regex string like r'^first/second/third/$' and then hook that to a View.

For Example, suppose I have a RegExp like this :

String regexString = r'/api/\w+/\d+/';
RegExp regExp = new RegExp(regexString);
View view = new View(); // a view class i made and suppose that this view is hooked to that url

And a HttRequest point to /api/topic/1/ and that would match that regex, then i can rendered anything hook to that url.

The problem is, from the regex above, how do i know that \w+ and \d+ value is topic and 1.

Care to give me some pointers anyone? Thank you.

Answer 1

You need to put the parts you want to extract into groups so you can extract them from the match. This is achieved by putting a part of the pattern inside parentheses.

// added parentheses around \w+ and \d+ to get separate groups 
String regexString = r'/api/(\w+)/(\d+)/'; // not r'/api/\w+/\d+/' !!!
RegExp regExp = new RegExp(regexString);
var matches = regExp.allMatches("/api/topic/3/");

print("${matches.length}");       // => 1 - 1 instance of pattern found in string
var match = matches.elementAt(0); // => extract the first (and only) match
print("${match.group(0)}");       // => /api/topic/3/ - the whole match
print("${match.group(1)}");       // => topic  - first matched group
print("${match.group(2)}");       // => 3      - second matched group

however, the given regex would also match "/api/topic/3/ /api/topic/4/" as it is not anchored, and it would have 2 matches (matches.length would be 2) - one for each path, so you might want to use this instead:

String regexString = r'^/api/(\w+)/(\d+)/$';

This ensures that the regex is anchored exactly from beginning to the end of the string, and not just anywhere inside the string.