Quick Start
Tutorial
Tools & Languages
Examples
Reference
Book Reviews
RegexBuddy Instantly and accurately analyze and test any regular expression with RegexBuddy. Save yourself the headache of figuring out the differences between regex flavors. RegexBuddy knows all the regex features and regex flavors listed in this reference. RegexBuddy also explains and emulates many more subtle differences and idiosyncrasies that are omitted here to keep this reference readable. Get your own copy of RegexBuddy now

Regular Expression Reference: Named Groups and Backreferences

FeatureSyntaxDescriptionExample JGsoft .NET Java Perl PCRE PCRE2 PHP Delphi R JavaScript VBScript XRegExp Python Ruby std::regex Boost Tcl ARE POSIX BRE POSIX ERE GNU BRE GNU ERE Oracle XML XPath
Named capturing group (?<name>regex) Captures the text matched by "regex" into the group "name". The name can contain letters and numbers but must start with a letter. (?<x>abc){3} matches abcabcabc. The group x matches abc. YESYES75.107.0YES5.2.2YESYESnonoYESno1.9noECMA
1.42–1.62
nononononononono
Named capturing group (?'name'regex) Captures the text matched by "regex" into the group "name". The name can contain letters and numbers but must start with a letter. (?'x'abc){3} matches abcabcabc. The group x matches abc. YESYESno5.107.0YES5.2.2YESYESnononono1.9noECMA
1.42–1.62
nononononononono
Named capturing group (?P<name>regex) Captures the text matched by "regex" into the group "name". The name can contain letters and numbers but must start with a letter. (?P<x>abc){3} matches abcabcabc. The group x matches abc. YESnono5.10YESYESYESYESYESnonoYESYESnonononononononononono
Duplicate named group Any named group Two named groups can share the same name. (?<x>a)|(?<x>b) matches a or b. YESYES7 error5.106.7 optionoption5.2.0 optionoptionoptionn/an/aerrorerror1.9n/aECMA
1.42–1.62
n/an/an/an/an/an/an/an/a
Duplicate named group Any named group Named groups that share the same name are treated as one an the same group, so there are no pitfalls when using backreferences to that name. YESYESn/anonononononon/an/an/an/anon/anon/an/an/an/an/an/an/an/a
Duplicate named group Any named group If a regex has multiple groups with the same name, backreferences using that name point to the leftmost group in the regex with that name. nonon/ano6.7–8.33no5.2.0–5.5.9XE–XE62.14.0–3.0.2n/an/an/an/anon/anon/an/an/an/an/an/an/an/a
Duplicate named group Any named group If a regex has multiple groups with the same name, backreferences using that name point to the leftmost group with that name that has actually participated in the match attempt when the backreference is evaluated. nonon/a5.108.36YES5.6.9no3.1.3n/an/an/an/anon/aECMA
1.47–1.62
n/an/an/an/an/an/an/an/a
Duplicate named group Any named group If a regex has multiple groups with the same name, backreferences using that name point to the rightmost group with that name that appears to the left of the backreference in the regex. nonon/anonononononon/an/an/an/anon/aECMA
1.42–1.46
n/an/an/an/an/an/an/an/a
Duplicate named group Any named group If a regex has multiple groups with the same name, backreferences using that name can match the text captured by any group with that name that appears to the left of the backreference in the regex. nonon/anonononononon/an/an/an/a1.9n/anon/an/an/an/an/an/an/an/a
Named backreference \k<name> Substituted with the text matched by the named group "name". (?<x>abc|def)=\k<x> matches abc=abc or def=def, but not abc=def or def=abc. YESYES75.107.0YES5.2.2YESYESnonoYESno1.9noECMA
1.47–1.62
nononononononono
Named backreference \k'name' Substituted with the text matched by the named group "name". (?'x'abc|def)=\k'x' matches abc=abc or def=def, but not abc=def or def=abc. YESYESno5.107.0YES5.2.2YESYESnononono1.9noECMA
1.47–1.62
nononononononono
Named backreference \k{name} Substituted with the text matched by the named group "name". (?'x'abc|def)=\k{x} matches abc=abc or def=def, but not abc=def or def=abc. nonono5.107.2YES5.2.4YESYESnonononononoECMA
1.47–1.62
nononononononono
Named backreference \g{name} Substituted with the text matched by the named group "name". (?'x'abc|def)=\g{x} matches abc=abc or def=def, but not abc=def or def=abc. nonono5.107.2YES5.2.4YESYESnonononononoECMA
1.42–1.62
nononononononono
Named backreference (?P=name) Substituted with the text matched by the named group "name". (?P<x>abc|def)=(?P=x) matches abc=abc or def=def, but not abc=def or def=abc. YESnono5.10YESYESYESYESYESnononoYESnonononononononononono
Failed backreference Any named backreference Backreferences to groups that did not participate in the match attempt fail to match. (?<x>a)?\k<x> matches aa but fails to match b. YESnon‑ECMA75.10YESYESYESYESYESn/an/aignoredYES1.9n/aECMA
1.47–1.62
n/an/an/an/an/an/an/an/a
Nested backreference Any named backreference Backreferences can be used inside the group they reference. (?<x>a\k<x>?){3} matches aaaaaa. YESYES75.106.5YES5.1.3YESYESn/an/aignored2.4–3.4 fail1.9 failn/aECMA
1.42–1.62 error
n/an/an/an/an/an/an/an/a
Forward reference Any named backreference Backreferences can be used before the group they reference. (\k<x>?(?<x>a)){3} matches aaaaaa. YESYES7 error5.106.7YES5.2.0YESYESn/an/aerrorerror1.9 errorn/aECMA
1.42–1.62 error
n/an/an/an/an/an/an/an/a
Named capturing group Any named capturing group A number is a valid name for a capturing group. (?<17>abc){3} matches abcabcabc. The group named "17" matches abc. YESYES7 error5.10 error4.0–8.33error5.0.0–5.1.2XE–XE62.14.0–3.0.2n/an/aerrorerror1.9 errorn/aECMA
1.42–1.62
n/an/an/an/an/an/an/an/a
Named capturing group Any capturing group with a number as its name If the name of the group is a number, that becomes the group's name and the group's number. (?<17>abc|def)=\17 matches abc=abc or def=def, but not abc=def or def=abc. noYESn/an/anon/anononon/an/an/an/an/an/anon/an/an/an/an/an/an/an/a
Named backreference Any named backreference A number is a valid name for a backreference which then points to a group with that number as its name. (?<17>abc|def)=\k<17> matches abc=abc or def=def, but not abc=def or def=abc. YESYESn/an/a4.0–8.33n/a5.0.0–5.1.2XE–XE62.14.0–3.0.2n/an/an/an/an/an/aECMA
1.42–1.62 error
n/an/an/an/an/an/an/an/a
Named capturing group Any named capturing group A negative number is a valid name for a capturing group. (?<-17>abc){3} matches abcabcabc. The group named "-17" matches abc. errorerror7 error5.10 errorerrorerrorerrorerrorerrorn/an/aerrorerror1.9 errorn/aECMA
1.42–1.62
n/an/an/an/an/an/an/an/a
Named backreference Any named backreference A negative number is a valid name for a backreference which then points to a group with that negative number as its name. n/an/an/an/an/an/an/an/an/an/an/an/an/an/an/aECMA
1.42–1.62 error
n/an/an/an/an/an/an/an/a
FeatureSyntaxDescriptionExample JGsoft .NET Java Perl PCRE PCRE2 PHP Delphi R JavaScript VBScript XRegExp Python Ruby std::regex Boost Tcl ARE POSIX BRE POSIX ERE GNU BRE GNU ERE Oracle XML XPath

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 Reference
Introduction
Table of Contents
Quick Reference
Characters
Basic Features
Character Classes
Shorthands
Anchors
Word Boundaries
Quantifiers
Unicode
Capturing Groups & Backreferences
Named Groups & Backreferences
Special Groups
Mode Modifiers
Recursion & Balancing Groups
Replacement Reference
Characters
Matched Text & Backreferences
Context & Case Conversion
Conditionals
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