Recipes 2.9 and 2.10 illustrate capturing groups and backreferences. To be more precise: these recipes use numbered capturing groups and numbered backreferences. Each group automatically gets a number, which you use for the backreference.

Modern regex flavors support named capturing groups in addition to numbered groups. The only difference between named and numbered groups is your ability to assign a descriptive name, instead of being stuck with automatic numbers. Named groups make your regular expression more readable and easier to maintain. Inserting a capturing group into an existing regex can change the numbers assigned to all the capturing groups. Names that you assign remain the same.

Python was the first regular expression flavor to support named capture. It uses the syntax ‹(?P<name>regex)›. The name must consist of word characters matched by ‹\w›. ‹(?P<name>› is the group’s opening bracket, and ‹)› is the closing bracket.

The designers of the .NET Regex class came up with their own syntax for named capture, using two interchangeable variants. ‹(?<name>regex)› mimics Python’s syntax, minus the P. The name must consist of word characters matched by ‹\w›. ‹(?<name>› is the group’s opening bracket, and ‹)› is the closing bracket.

The angle brackets in the named capture syntax are annoying when you’re coding in XML, or writing this book in DocBook XML. That’s the reason for .NET’s alternate named capture syntax: ‹(?'name'regex)›. The angle brackets are replaced with single quotes. Choose whichever syntax is easier for you to type. Their functionality is identical.

Perhaps due to .NET’s popularity over Python, the .NET syntax seems to be the one that other regex library developers prefer to copy. Perl 5.10 and later have it, and so does the Oniguruma engine in Ruby 1.9. Perl 5.10 and Ruby 1.9 support both the syntax using angle brackets and single quotes. Java 7 also copied the .NET syntax, but only the variant using angle brackets. Standard JavaScript does not support named capture. XRegExp adds support for named capture using the .NET syntax, but only the variant with angle brackets.

PCRE copied Python’s syntax long ago, at a time when Perl did not support named capture at all. PCRE 7, the version that adds the new features in Perl 5.10, supports both the .NET syntax and the Python syntax. Perhaps as a testament to the success of PCRE, in a reverse compatibility move, Perl 5.10 also supports the Python syntax. In PCRE and Perl 5.10, the functionality of the .NET syntax and the Python syntax for named capture is identical.

Choose the syntax that is most useful to you. If you’re coding in PHP and you want your code to work with older versions of PHP that incorporate older versions of PCRE, use the Python syntax. If you don’t need compatibility with older versions and you also work with .NET or Ruby, the .NET syntax makes it easier to copy and paste between all these languages. If you’re unsure, use the Python syntax for PHP/PCRE. People recompiling your code with an older version of PCRE are going to be unhappy if the regexes in your code suddenly stop working. When copying a regex to .NET or Ruby, deleting a few Ps is easy enough.

Documentation for PCRE 7 and Perl 5.10 barely mention the Python syntax, but it is by no means deprecated. For PCRE and PHP, we actually recommend it.

With named capture comes named backreferences. Just as named capturing groups are functionally identical to numbered capturing groups, named backreferences are functionally identical to numbered backreferences. They’re just easier to read and maintain.

Python uses the syntax ‹(?P=name)› to create a backreference to the group name. Although this syntax uses parentheses, the backreference is not a group. You cannot put anything between the name and the closing parenthesis. A backreference ‹(?P=name)› is a singular regex token, just like ‹\1›. PCRE and Perl 5.10 also support the Python syntax for named backreferences.

.NET uses the syntax ‹\k<name>› and ‹\k'name'›. The two variants are identical in functionality, and you can freely mix them. A named group created with the bracket syntax can be referenced with the quote syntax, and vice versa. Perl 5.10, PCRE 7, and Ruby 1.9 also support the .NET syntax for named backreferences. Java 7 and XRegExp support only the variant using angle brackets.

We strongly recommend you don’t mix named and numbered groups in the same regex. Different flavors follow different rules for numbering unnamed groups that appear between named groups. Perl 5.10, Ruby 1.9, Java 7, and XRegExp copied .NET’s syntax, but they do not follow .NET’s way of numbering named capturing groups or of mixing numbered capturing groups with named groups. Instead of trying to explain the differences, we simply recommend not mixing named and numbered groups. Avoid the confusion and either give all unnamed groups a name or make them noncapturing.

Perl 5.10, Ruby 1.9, and .NET allow multiple named capturing groups to share the same name. We take advantage of this in the solutions for recipes 4.5, 8.7, and 8.19. When a regular expression uses alternation to find different variations of certain text, using capturing groups with the same name makes it easy to extract parts from the match, regardless of which alternative actually matched the text. The section Pure regular expression in Recipe 4.5 uses alternation to separately match dates in months of different lengths. Each alternative matches the day and the month. By using the same group names “day” and “month” in all the alternatives, we only need to query two capturing groups to retrieve the day and the month after the regular expression finds a match.

All the other flavors in this book that support named capture treat multiple groups with the same name as an error.