Boost Regex - Alternation documentation example
According to the Boost Regex documentation for Alternation (http://www.boost.org/doc/libs/1_64_0/libs/regex/doc/html/boost_regex/syntax/...) Quoting a piece of the documentation: ----- |abc is not a valid expression, but (?:)|abc is and is equivalent, also the expression: (?:abc)?? has exactly the same effect. ----- Is the last expression: (?:abc)?? correct? Why does it need two '?' operators instead of only one? If I try this expression with the input "abc" the Boost engine does not match "abc" as I'd have expected. However the expression: (?:abc)? matches as expected. Nick
On 15/07/2017 14:12, Nick via Boost-users wrote:
According to the Boost Regex documentation for Alternation (http://www.boost.org/doc/libs/1_64_0/libs/regex/doc/html/boost_regex/syntax/...)
Quoting a piece of the documentation:
-----
|abc is not a valid expression, but
(?:)|abc is and is equivalent, also the expression:
(?:abc)?? has exactly the same effect.
-----
Is the last expression:
(?:abc)??
correct?
Yes.
Why does it need two '?' operators instead of only one?
Non-greedy, matches nothing for preference, otherwise abc, same as "(?:)|abc" does.
If I try this expression with the input "abc" the Boost engine does not match "abc" as I'd have expected. However the expression:
(?:abc)?
matches as expected.
That has the opposite semantics: abc as first choice, nothing as second. ie the same as "abc|(?:)". HTH, John. --- This email has been checked for viruses by AVG. http://www.avg.com
Somehow I didn't get this reply for a while. On Sat, 2017-07-15 at 15:44 +0100, John Maddock via Boost-users wrote:
On 15/07/2017 14:12, Nick via Boost-users wrote:
Why does it need two '?' operators instead of only one?
Non-greedy, matches nothing for preference, otherwise abc, same as "(?:)|abc" does.
Good to know, thanks. Makes me wonder what scenario(s) that would be useful for
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John Maddock -
Nick