Re: Need help with regular expression to parse URLs

Tom Anderson <>
Mon, 10 Aug 2009 22:48:25 +0100
On Mon, 10 Aug 2009, markspace wrote:

Neil wrote:

I wrote this regular expression:

It seems to be working fine for most urls, but it barfed on this one:

The matcher gives me 1 group with this value: s/Backpacks

I dont understand how that could have happened. I was expecting to
two groups:

Any ideas what went wrong?

You have two problems.

Firstly, the repeated group as written has no way to admit slashes
*between* pairs of path elements. Expand the repetition by hand (three
times, here):


You get the slash between elements in a pair, but not between pairs. This
explains your results. You need something that expands to:




You can get the individual elements with smaller capturing groups (here
making the pair-level group non-capturing):


Secondly, you get one matching group per occurrence of a capturing group
in the *pattern*, not per occurrence of the subpattern in the match. That
is, if the above pair group matches five times, you'll still only get a
single pair of captured groups (the last ones). That, i think, means
there's no way to use a regular expression to do what you want to do here.

At least, not directly. What you can do is make a regexp which matches a
single occurrence of a pair of elements, and then use the Matcher's find()
method to loop over all occurrences in the string. Like so:

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

public class Split {
  public static void main(String... args) throws URISyntaxException {
  Pattern whole = Pattern.compile("^/jamm/[^/]+/[^/]+(.*?)\\.html?$");
  Pattern pair = Pattern.compile("([^/]+)/([^/]+)");
  for (String s: args) {
  URI uri = new URI(s);
  String path = uri.getPath();
  Matcher wholeMatch = whole.matcher(path);
  if (wholeMatch.matches()) {
  Matcher pairMatch = pair.matcher(;
  while (pairMatch.find()) {
  String first =;
  String second =;
  System.out.println(Integer.toString(pairMatch.start()) + "\t" + first + "\t" + second);

Note that rather than matching against the raw URL string, i'm going via; this saves me having to match the other bits of the URL
explicitly, and also takes care of resolving % escapes.

I don't understand what the * was in the end of your regex: "*\.html" ?

It's a quantifier on the preceding group - the one which captures the
paired path components like 'Stuff/Bags-%26-Luggage'. It means that there
can be any number of such pairs.


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