Quick Start
Tutorial
Tools & Languages
Examples
Reference
Book Reviews
RegexBuddy—Better than a regular expression tutorial!

Relative Backreferences

Some applications support relative backreferences. These use a negative number to reference a group preceding the backreference. To find the group that the relative backreference refers to, take the absolute number of the backreference and count that many opening parentheses of (named or unnamed) capturing groups starting at the backreference and going from right to left through the regex. So (a)(b)(c)\k<-1> matches abcc and (a)(b)(c)\k<-3> matches abca. If the backreference is inside a capturing group, then you also need to count that capturing group's opening parenthesis. So (a)(b)(c\k<-2>) matches abcb. (a)(b)(c\k<-1>) either fails to match or is an error depending on whether your application allows nested backreferences.

The syntax for nested backreferences varies widely. It is generally an extension of the syntax for named backreferences. JGsoft V2 and Ruby 1.9 and later support \k<-1> and \k'-1'. Though this looks like the .NET syntax for named capture, .NET itself does not support relative backreferences.

Perl 5.10, PCRE 7.0, PHP 5.2.2, and R support \g{-1} and \g-1.

Boost supports the Perl syntax starting with Boost 1.42. Boost adds the Ruby syntax starting with Boost 1.47. To complicate matters, Boost 1.47 allowed these variants to multiply. Boost 1.47 and later allow relative backreferences to be specified with \g or \k and with curly braces, angle brackets, or quotes. That makes six variations plus \g-1 for a total of seven variations. This puts Boost in conflict with Ruby, PCRE, PHP, R, and JGsoft which treat \g with angle brackets or quotes and a negative number as a relative subroutine call.

Make a Donation

Did this website just save you a trip to the bookstore? Please make a donation to support this site, and you'll get a lifetime of advertisement-free access to this site! Credit cards, PayPal, and Bitcoin gladly accepted.

Regex Tutorial
Introduction
Table of Contents
Special Characters
Non-Printable Characters
Regex Engine Internals
Character Classes
Character Class Subtraction
Character Class Intersection
Shorthand Character Classes
Dot
Anchors
Word Boundaries
Alternation
Optional Items
Repetition
Grouping & Capturing
Backreferences
Backreferences, part 2
Named Groups
Relative Backreferences
Branch Reset Groups
Free-Spacing & Comments
Unicode
Mode Modifiers
Atomic Grouping
Possessive Quantifiers
Lookahead & Lookbehind
Lookaround, part 2
Keep Text out of The Match
Conditionals
Balancing Groups
Recursion
Subroutines
Infinite Recursion
Recursion & Quantifiers
Recursion & Capturing
Recursion & Backreferences
Recursion & Backtracking
POSIX Bracket Expressions
Zero-Length Matches
Continuing Matches
More on This Site
Introduction
Regular Expressions Quick Start
Regular Expressions Tutorial
Replacement Strings Tutorial
Applications and Languages
Regular Expressions Examples
Regular Expressions Reference
Replacement Strings Reference
Book Reviews
Printable PDF
About This Site
RSS Feed & Blog