Question

I need to pull a variable out of a URL or get an empty string if that variable is not present.

Pseudo code:

String foo = "http://abcdefg.hij.klmnop.com/a/b/c.file?foo=123&zoo=panda";
String bar = "http://abcdefg.hij.klmnop.com/a/b/c.file";

when I run my regex I want to get 123 in the first case and empty string in the second.

I’m trying this as my replace .*?foo=(.*?)&?.*
replacing this with $1 but that’s not working when foo= isn’t present.

I can’t just do a match, it has to be a replace.

Answer 1

You can try this:

[^?]+(?:\?foo=([^&]+).*)?

If there are parameters and the first parameter is named “foo”, its value will be captured in group #1. If there are no parameters the regex will still succeed, but I can’t predict what will happen when you access the capturing group. Some possibilities:

• it will contain an empty string
• it will contain a null reference, which will be automatically converted to
  • an empty string
  • the word “null”
• your app will throw an exception because group #1 didn’t participate in the match.

This regex matches the sample strings you provided, but it won’t work if there’s a parameter list that doesn’t include “foo”, or if “foo” is not the first parameter. Those options can be accommodated too, assuming the capturing group thing works.