Java RegEx Pattern not matching (works in .NET)

Question

Below is an example that is producing an exception in Java (and not matching the input). Perhaps I misunderstood the JavaDoc, but it looks like it should work. Using the same pattern and input in C# will produce a match.

import java.util.regex.Matcher;
import java.util.regex.Pattern;

public class Main { 
    public static void main(String[] args) {
        String pattern = "aspx\\?uid=([^']*)";      
        Pattern p = Pattern.compile(pattern);
        Matcher m = p.matcher("id='page.aspx?uid=123'");
        System.out.println(m.groupCount() > 0 ? m.group(1) : "No Matches");     
    }
}

EDIT: I should note the JavaDoc says the following about Matcher.groupCount

Any non-negative integer smaller than or equal to the value returned by this method is guaranteed to be a valid group index for this matcher.

Answer 1

It's throwing an exception because the pattern didn't match but you tried to get a group from it (m.matches() would be false here); groupCount() will return the number of groups that would be in a match, regardless of if there actually was one. As for why the match isn't working, Java Patterns match on the entire string, not on a substring

Answer 2

Try calling

m.find();

after the .matcher statement.