impulse9 wrote:
Hi All,
I've just started to use boost::regex I met with the following incompatibility with Perl regular expressions:
It is impossible to use empty expressions, neither alone nor inside of another regex statement.
Sample code:
boost::regex e("(\\d+)\\s*(kbit|)");
From the Boost.Regex docs (http://boost.org/libs/regex/doc/syntax_perl.html):
Empty alternatives are not allowed (these are almost always a mistake), but if you really want an empty alternative use (?:) as a placeholder, for example:
"|abc" is not a valid expression, but "(?:)|abc" is and is equivalent, also the expression: "(?:abc)??" has exactly the same effect.
This would seem to be non-compliant behavior according to TR1, which references ECMA-262, which describes the regex syntax in section 15.10.1 as:
Pattern :: Disjunction Disjunction :: Alternative Alternative | Disjunction Alternative :: [empty] Alternative Term
The TR1 draft (http://open-std.org/JTC1/SC22/WG21/docs/papers/2005/n1836.pdf) amends ECMA-262 in a few ways, but says nothing that would indicate that empty alternatives are not allowed. FWIW, you could use Boost.Xpressive (new in 1.34), which allows empty alternates. -- Eric Niebler Boost Consulting www.boost-consulting.com