Regex Named Groups in Java

(Update: August 2011)

As geofflane mentions in his answer, Java 7 now support named groups.
tchrist points out in the comment that the support is limited.
He details the limitations in his great answer “Java Regex Helper“

Java 7 regex named group support was presented back in September 2010 in Oracle’s blog.

In the official release of Java 7, the constructs to support the named capturing group are:

• (?<name>capturing text) to define a named group “name”
• \k<name> to backreference a named group “name”
• ${name} to reference to captured group in Matcher’s replacement string
• Matcher.group(String name) to return the captured input subsequence by the given “named group”.

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Other alternatives for pre-Java 7 were:

• Google named-regex (see John Hardy‘s answer)
  Gábor Lipták mentions (November 2012) that this project might not be active (with several outstanding bugs), and its GitHub fork could be considered instead.
• jregex (See Brian Clozel‘s answer)

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(Original answer: Jan 2009, with the next two links now broken)

You can not refer to named group, unless you code your own version of Regex…

That is precisely what Gorbush2 did in this thread.

Regex2

(limited implementation, as pointed out again by tchrist, as it looks only for ASCII identifiers. tchrist details the limitation as:

only being able to have one named group per same name (which you don’t always have control over!) and not being able to use them for in-regex recursion.

Note: You can find true regex recursion examples in Perl and PCRE regexes, as mentioned in Regexp Power, PCRE specs and Matching Strings with Balanced Parentheses slide)

Example:

String:

"TEST 123"

RegExp:

"(?<login>\\w+) (?<id>\\d+)"

Access

matcher.group(1) ==> TEST
matcher.group("login") ==> TEST
matcher.name(1) ==> login

Replace

matcher.replaceAll("aaaaa_$1_sssss_$2____") ==> aaaaa_TEST_sssss_123____
matcher.replaceAll("aaaaa_${login}_sssss_${id}____") ==> aaaaa_TEST_sssss_123____ 

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(extract from the implementation)

public final class Pattern
    implements java.io.Serializable
{
[...]
    /**
     * Parses a group and returns the head node of a set of nodes that process
     * the group. Sometimes a double return system is used where the tail is
     * returned in root.
     */
    private Node group0() {
        boolean capturingGroup = false;
        Node head = null;
        Node tail = null;
        int save = flags;
        root = null;
        int ch = next();
        if (ch == '?') {
            ch = skip();
            switch (ch) {

            case '<':   // (?<xxx)  look behind or group name
                ch = read();
                int start = cursor;
[...]
                // test forGroupName
                int startChar = ch;
                while(ASCII.isWord(ch) && ch != '>') ch=read();
                if(ch == '>'){
                    // valid group name
                    int len = cursor-start;
                    int[] newtemp = new int[2*(len) + 2];
                    //System.arraycopy(temp, start, newtemp, 0, len);
                    StringBuilder name = new StringBuilder();
                    for(int i = start; i< cursor; i++){
                        name.append((char)temp[i-1]);
                    }
                    // create Named group
                    head = createGroup(false);
                    ((GroupTail)root).name = name.toString();

                    capturingGroup = true;
                    tail = root;
                    head.next = expr(tail);
                    break;
                }