Alternation Constructs

The following table lists special characters that modify a regular expression to allow either/or matching.

Alternation construct	Definition
\|	Matches any one of the terms separated by the \| (vertical bar) character; for example, `cat\|dog\|tiger`. The leftmost successful match wins.
(?(expression)yes\|no)	Matches the "yes" part if the expression matches at this point; otherwise, matches the "no" part. The "no" part can be omitted. The expression can be any valid subexpression, but it is turned into a zero-width assertion, so this syntax is equivalent to (?(?=expression)yes\|no). Note that if the expression is the name of a named group or a capturing group number, the alternation construct is interpreted as a capture test (described in the next row of this table). To avoid confusion in these cases, you can spell out the inside (?=expression) explicitly.
(?(name)yes\|no)	Matches the "yes" part if the named capture string has a match; otherwise, matches the "no" part. The "no" part can be omitted. If the given name does not correspond to the name or number of a capturing group used in this expression, the alternation construct is interpreted as an expression test (described in the preceding row of this table).

Alternation construct	Definition
\|	Matches any one of the terms separated by the \| (vertical bar) character; for example, `cat\|dog\|tiger`. The leftmost successful match wins.
(?(expression)yes\|no)	Matches the "yes" part if the expression matches at this point; otherwise, matches the "no" part. The "no" part can be omitted. The expression can be any valid subexpression, but it is turned into a zero-width assertion, so this syntax is equivalent to (?(?=expression)yes\|no). Note that if the expression is the name of a named group or a capturing group number, the alternation construct is interpreted as a capture test (described in the next row of this table). To avoid confusion in these cases, you can spell out the inside (?=expression) explicitly.
(?(name)yes\|no)	Matches the "yes" part if the named capture string has a match; otherwise, matches the "no" part. The "no" part can be omitted. If the given name does not correspond to the name or number of a capturing group used in this expression, the alternation construct is interpreted as an expression test (described in the preceding row of this table).

See Also