Regular expression with variable number of groups?

Is it possible to create a regular expression with a variable number of groups?

After running this for instance...

Pattern p = Pattern.compile("ab([cd])*ef");
Matcher m = p.matcher("abcddcef");
m.matches();

... I would like to have something like

• m.group(1) = "c"
• m.group(2) = "d"
• m.group(3) = "d"
• m.group(4) = "c".

(Background: I'm parsing some lines of data, and one of the "fields" is repeating. I would like to avoid a matcher.find loop for these fields.)

As pointed out by @Tim Pietzcker in the comments, perl6 and .NET have this feature.

Solution 1:

According to the documentation, Java regular expressions can't do this:

The captured input associated with a group is always the subsequence that the group most recently matched. If a group is evaluated a second time because of quantification then its previously-captured value, if any, will be retained if the second evaluation fails. Matching the string "aba" against the expression (a(b)?)+, for example, leaves group two set to "b". All captured input is discarded at the beginning of each match.

(emphasis added)

Solution 2:

You can use split to get the fields you need into an array and loop through that.

http://download.oracle.com/javase/1,5.0/docs/api/java/lang/String.html#split(java.lang.String)

Solution 3:

I have not used java regex, but for many languages the answer is: No.

Capturing groups seem to be created when the regex is parsed, and filled when it matches the string. The expression (a)|(b)(c) has three capturing groups, only if either one, or two of them can be filled. (a)* has just one group, the parser leaves the last match in the group after matching.

Solution 4:

Pattern p = Pattern.compile("ab(?:(c)|(d))*ef");
Matcher m = p.matcher("abcdef");
m.matches();

should do what you want.

EDIT:

@aioobe, I understand now. You want to be able to do something like the grammar

A    ::== <Foo> <Bars> <Baz>
Foo  ::== "foo"
Baz  ::== "baz"
Bars ::== <Bar> <Bars>
        | ε
Bar  ::== "A"
        | "B"

and pull out all the individual matches of Bar.

No, there is no way to do that using java.util.regex. You can recurse and use a regex on the match of Bars or use a parser generator like ANTLR and attach a side-effect to Bar.