How to get the AST of a regular expression string?

Question

How can I get the abstract syntax tree (AST) of a regular expression (in C++)?

For example,

 (XYZ)|(123)

should yield a tree of:<

Answer 1

boost::regex seems to have a hand-written recursive-descent parser in basic_regex_parser.hpp. Even though it feels awfully like re-inventing the wheel, you are probably faster when writing up the grammar in boost::spirit yourself, especially with the multitude of regex formats around.