Get a substring from a string using regex

Question

I have many strings in this format:

fdg.sdfg.234fdsa.dsf_1.2.5.62.xml
23432ssdfsa_sadfsd_1.2.7.6.xml
3.3.3asdf_ddd_1.2.1.doc

I would like to get only the number
from: fdg.sdfg.234fdsa.dsf_1.2.5.62.xml to get: 1.2.5.62
from: f23432ssdfsa_sadfsd_1.2.7.6.xml to get: 1.2.7.6
from: f3.3.3asdf_ddd_1.2.1.doc to get: 1.2.1
etc

This code works:

string test = "4534534ghgggg_1.1.3.4.xml";
int to = test.LastIndexOf('.');
int from = test.LastIndexOf('_') + 1;
Console.WriteLine(test.Substring(from,to - from));

But I want to know how can I do it with regex. Any ideas?

Answer 1

First, let's elaborate the rules ( number is not you want to get) for the match:

• starts with '_' (not included in match)
• contains digits and dots (dots are not duplcated).
• no leading and no trailing dots are allowed
• has at least one digit as well as at least one dot
• ends with '.' (not included in match)

then implement a pattern:

 (?<=_)[0-9]+(\.[0-9]+)+(?=\.)

if the number in the question is, in fact, some kind of version you may want to restict number of its parts, eg

 (?<=_)[0-9]+(\.[0-9]+){1,3}(?=\.[^0-9])

which means that only 2 to 4 parts versions ( _d.d. , _d.dd and _d.ddd ) are accepted. Eg input _1.2.15. will be accepted (3 parts: 1 , 2 and 15 ) when _1.2.3.4.5. will be rejected (5 parts)

finally, use regular expressions:

  string source = ...
  string pattern = @"(?<=_)[0-9]+(\.[0-9]+)+(?=\.)";

  // If there are many matches, let's take the last one
  string lastMatch = Regex.Matches(pattern, source)
    .OfType<Match>()
    .Select(match => match.Value)
    .LastOrDefault();

  Console.Write(lastMatch); 

However, if format is fixed then regular expression (and Linq ) is overshoot. LastIndex + Substring is a better choice.

Answer 2

This code seems to work as long as the numbers you are looking for are preceded by "_".

Edited - This is the final working result

        // fdg.sdfg.234fdsa.dsf_1.2.5.62.xml 
        // 23432ssdfsa_sadfsd_1.2.7.6.xml
        // 3.3.3asdf_ddd_1.2.1.doc

        string source = "fdg.sdfg.234fdsa.dsf_1.2.5.62.xml";
         var match = Regex.Match(source, @"_[0-9]+\.[0-9]+\.[0-9]+(\.[0-9]+)*").ToString().Replace("_", "");
        Console.WriteLine(match);
        Console.ReadLine();

Answer 3

You already got all your answers. I have not practised for the last 6 months and have almost all forgotten. Anyway there are plenty of web sites ( look for regex tester in your favorite search engine ) that helps you with regex. I do not know if I can mention one more than the other but here are some snapshots of one example ( I am not the latest expert in regex so I hope I did not write something too wrong).

So now you can test all the answers and advices that have been brought to you. .