How would you use a regular expression to ignore strings that contain a specific substring?

Question

How would I go about using a negative lookbehind(or any other method) regular expression to ignore strings that contains a specific substring?

I've read two previous stackoverflow questions:
java-regexp-for-file-filtering
regex-to-match-against-something-that-is-not-a-specific-substring

They are nearly what I want... my problem is the string doesn't end with what I want to ignore. If it did this would not be a problem.

I have a feeling this has to do with the fact that lookarounds are zero-width and something is matching on the second pass through the string... but, I'm none too sure of the internals.

Anyway, if anyone is willing to take the time and explain it I will greatly appreciate it.

Here is an example of an input string that I want to ignore:

192.168.1.10 - - [08/Feb/2009:16:33:54 -0800] "GET /FOO/BAR/ HTTP/1.1" 200 2246

Here is an example of an input string that I want to keep for further evaluation:

192.168.1.10 - - [08/Feb/2009:16:33:54 -0800] "GET /FOO/BAR/content.js HTTP/1.1" 200 2246

The key for me is that I want to ignore any HTTP GET that is going after a document root default page.

Following is my little test harness and the best RegEx I've come up with so far.

public static void main(String[] args){
String inString = "192.168.1.10 - - [08/Feb/2009:16:33:54 -0800] \"GET /FOO/BAR/ HTTP/1.1\" 200 2246";
//String inString = "192.168.1.10 - - [08/Feb/2009:16:33:54 -0800] \"GET /FOO/BAR/content.js HTTP/1.1\" 200 2246";
//String inString = "192.168.1.10 - - [08/Feb/2009:16:33:54 -0800] \"GET /FOO/BAR/content.js HTTP/"; // This works
//String inString = "192.168.1.10 - - [08/Feb/2009:16:33:54 -0800] \"GET /FOO/BAR/ HTTP/"; // This works
String inRegEx = "^.*(?:GET).*$(?<!.?/ HTTP/)";
try {
  Pattern pattern = Pattern.compile(inRegEx);

  Matcher matcher = pattern.matcher(inString);

  if (matcher.find()) {
    System.out.printf("I found the text \"%s\" starting at " +
"index %d and ending at index %d.%n",
matcher.group(), matcher.start(), matcher.end());
  } else {
    System.out.printf("No match found.%n");
  }
} catch (PatternSyntaxException pse) {
  System.out.println("Invalid RegEx: " + inRegEx);
  pse.printStackTrace();
}
}

Answer 1

Could you just match any path that doesn't end with a /

String inRegEx = "^.* \"GET (.*[^/]) HTTP/.*$";

---

This can also be done using negative lookbehind

String inRegEx = "^.* \"GET (.+)(?<!/) HTTP/.*$";

Here, (?<!/) says "the preceding sequence must not match /".

Answer 2

I would use something like this:

"\"GET /FOO/BAR/[^ ]+ HTTP/1\.[01]\""

This matches every path that’s not just /FOO/BAR/.

Answer 3

If you are writing Regex this complex, I would recommend building a library of resources outside of StackOverflow.

• Java Regular Expressions by Mehran Habibi (Apress)
• Mastering Regular Expressions, Second Edition
• RegExLib

Answer 4

Maybe I'm missing something here, but couldn't you just go without any regular expression and ignore anything for which this is true:

string.contains("/ HTTP")

Because a file path will never end with a slash.