Feature: Conditional Expressions (original) (raw)

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A Conditional Expression checks a condition and evaluates its first alternative if the condition is true; otherwise, it evaluates its second alternative.

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.

• (?(_condition_)_yes-pattern_|_no-pattern_) — Matches yes-pattern if condition is true; otherwise, matches no-pattern.
• (?(_condition_)_yes-pattern_) — Matches yes-pattern if condition is true; otherwise, matches the empty string.

Conditions

The following conditions are supported:

• (?(?=_test-pattern_)…) — Evaluates to true if a lookahead for test-pattern matches; Otherwise, evaluates to false.
• (?(?!_test-pattern_)…) — Evaluates to true if a negative lookahead for test-pattern matches; Otherwise, evaluates to false.
• (?(_n_)…) — Evaluates to true if the capture group at offset n was successfully matched; Otherwise, evaluates to false.
• (?(<_name_>)…) — Evaluates to true if the named capture group with the name name was successfully matched; Otherwise, evaluates to false.
• (?('_name_')…) — Evaluates to true if the named capture group with the name name was successfully matched; Otherwise, evaluates to false.
• (?(R)…) — Evaluates to true if inside a recursive expression; Otherwise, evaluates to false.
• (?(R_n_)…) — Evaluates to true if inside a recursive expression for the capture group at offset n; Otherwise, evaluates to false.
• (?(R&_name_)…) — Evaluates to true if inside a recursive expression for the named capture group with the name name; Otherwise, evaluates to false.
• (?(DEFINE)…) — Always evaluates to false. This allows you to define Subroutines.

Engines