How do I use named capturing groups in Java regular expressions?

Regular named groups in Java

Background introduction

Java’s java.regex package does not support named groups, so this article will introduce third-party libraries to resolve this issue.

Third-party libraries that support named groups

Java 7

Starting from Java 7, Java natively supports named groups. You can use the following structure:

• (?capturing text): defines a named group named "name"
• k: refers to a named group named " name"'s named group
• ${name}: Reference the captured group in Matcher's replacement string
• Matcher.group(String name): Return by the given "named group" Captured input subsequence.
  Please refer to Oracle blog for details.

Pre-Java 7 Alternatives

For versions prior to Java 7, the following third-party libraries provide named group support:

• Google named-regex: Has full support, but there may be some bugs to be resolved.
• jregex: Limited support, only for ASCII identifiers.

Named Group Example

The following is an example of using named groups:

String:

"TEST 123"

Regular expression:

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

Access:

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

Replacement:

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

Limitations and Notes

• Java 7 still has limited support for named groups (see tchrist's answer for details).
• Some libraries' named groups may not work with regex recursion.
• When specifying named groups for regular expressions, naming conventions need to be strictly adhered to (e.g., ASCII identifiers only).

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