Regex Named Groups in Java

Question

It is my understanding that the java.regex package does not have support for named groups (http://www.regular-expressions.info/named.html) so can anyone point me towards a third-party library that does?

I've looked at jregex but its last release was in 2002 and it didn't work for me (admittedly I only tried briefly) under java5.

Answer 1

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

---

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____

---

(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;
                }

Answer 2

Yes but its messy hacking the sun classes. There is a simpler way:

http://code.google.com/p/named-regexp/

named-regexp is a thin wrapper for the standard JDK regular expressions implementation, with the single purpose of handling named capturing groups in the .net style : (?...).

It can be used with Java 5 and 6 (generics are used).

Java 7 will handle named capturing groups , so this project is not meant to last.

Answer 3

What kind of problem do you get with jregex? It worked well for me under java5 and java6.

Jregex does the job well (even if the last version is from 2002), unless you want to wait for javaSE 7.