Backreferences in Java Regular Expressions is another important feature provided by Java.

To understand backreferences, we need to understand group first. Group in regular expression means treating multiple characters as a single unit. They are created by placing the characters to be grouped inside a set of parentheses – ”()”. Each set of parentheses corresponds to a group.

Backreferences are convenient, because it allows us to repeat a pattern without writing it again. We can just refer to the previous defined group by using \#(# is the group number). This will make more sense after you read the following two examples.

Example 1: Finding Repeated Pattern

(\d\d\d)\1 matches 123123, but does not match 123456 in a row. This indicates that the referred pattern needs to be exactly the name.

String str = "123456";
Pattern p = Pattern.compile("(\\d\\d\\d)\\1");
Matcher m = p.matcher(str);
System.out.println(m.groupCount());
while (m.find()) {
String word = m.group();
System.out.println(word + " " + m.start() + " " + m.end());
}

1
123123 0 6

Example 2: Finding Duplicate Words

String pattern = "\\b(\\w+)\\b[\\w\\W]*\\b\\1\\b";
Pattern p = Pattern.compile(pattern, Pattern.CASE_INSENSITIVE);
String phrase = "unique is not duplicate but unique, Duplicate is duplicate.";
Matcher m = p.matcher(phrase);
while (m.find()) {
String val = m.group();
System.out.println("Matching subsequence is \"" + val + "\"");
System.out.println("Duplicate word: " + m.group(1) + "\n");
}

Matching subsequence is “unique is not duplicate but unique”
Duplicate word: unique

Matching subsequence is “Duplicate is duplicate”
Duplicate word: Duplicate

Note: This is not a good method to use regular expression to find duplicate words. From the example above, the first “duplicate” is not matched.

Why Use Backreferences?

Check out more regular expression examples.