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.

Chenault

Your understanding is incorrect. JDK7 handles named groups. – Formwork

@Formwork In 2009 there was no JDK7. – Municipal

Note: this is discussed on Meta, 12 years later. – Chartres

Answer 1

(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".

---

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)

---

(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____ 

---

(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;
                }

Chartres

both links above seems to be broken? – Rainband

This code is buggy. It is looking for ASCII identifiers. That’s wrong. It should be looking for anything that Java allows in an identifier!! – Formwork

Just FYI since you seem so conscientious, the limited part isn’t so much about the ASCII vs Unicode names as it is about 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. – Formwork

There is no matcher.name(int index) method for Matcher object in Java ?? – Cockiness

@ot0 At least not in the JDK I was considering 10 years ago... Nowadays, I wouldn't know. I don't see one in their current up-to-date tutorial: docs.oracle.com/javase/tutorial/essential/regex/matcher.html – Chartres

Answer 2

For people coming to this late: Java 7 adds named groups. Matcher.group(String groupName) documentation.

Prurient

Answer 3

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.

Spiculum

Too bad this cant be used from within GWT. – Colston

Check out the GitHub fork of this project, which fixes several bugs from the original. It's also hosted in Maven Central. – Chew

Just a word of caution in my case, the tony19 fork on Github doesn't work on Android as of 0.1.8. – Hover

@RubberMallet, The Android-specific problem is now fixed and will be in 0.1.9. – Chew

Answer 4

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.

Dessiatine

Answer 5

For those running pre-java7, named groups are supported by joni (Java port of the Oniguruma regexp library). Documentation is sparse, but it has worked well for us.
Binaries are available via Maven (http://repository.codehaus.org/org/jruby/joni/joni/).

Magocsi

I am very interested in the joni option mentioned by Ryan above -- do you have any code snippets using named capture groups - I have managed to get basic matching and searching to work correctly - but I don't see which method I would use to get access to the groupNames or to get a capture's value using the group name. – Masters

Answer 6

A bit old question but I found myself needing this also and that the suggestions above were inaduquate - and as such - developed a thin wrapper myself: https://github.com/hofmeister/MatchIt

Sanction