Feature: Subroutines

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A Subroutine is a pre-defined capture group or named capture group that can be reused in multiple places within the pattern to re-evaluate the subexpression from the group.

Syntax

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NOTE: The following syntax is an example based on some of the supported engines. For specific engine support, see Engines.

(?(DEFINE)…) — Defines a set of reusable capture groups that can be referenced elsewhere in the pattern.
(?n) — Where n is an integer >= 1. Evaluates the capture group whose offset is n.
(?-n) — Where n is an integer >= 1. Evaluates the capture group whose offset is the nth capture group declared to the left of the current Atom. Example: (?-1) would revaluate the last declared capture group.
(?+n) — Where n is an integer >= 1. Evaluates the capture group whose offset is the nth capture group declared to the right of the current Atom. Example: (?+1) would evaluate the next declared capture group.
(?&name) — Evaluates the named capture group with the provided name.

Example

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(?(DEFINE)
  (?<Year>\d{4}|[+-]\d{5,})
  (?<Month>0[1-9]|1[0-2])
  (?<Day>0[1-9]|2[0-9]|3[01])
)
(?<Date>(?&Year)-(?&Month)-(?&Day)|(?&Year)(?&Month)(?&Day))

Engines

Engine	Supported
Perl	✔
PCRE	✔
Boost.Regex	✔
.NET	❌
Oniguruma	✔
Hyperscan	❌
ICU	❌
Glib/GRegex	✔
ECMAScript	❌