Is it okay to pass a temporary regex object to regex_match?

Question

Most examples using boost::regex_match construct the boost::regex object before calling match; e.g.

boost::regex pattern("(...)");
boost::smatch match;
if (boost::regex_match(text, match, pattern)) {
    // use the match object here
}

(See https://github.com/boostorg/regex/tree/develop/example/snippets for other examples).

However, suppose I instead pass a temporary regex object to regex_match, e.g.

boost::smatch match;
if (boost::regex_match(text, match, boost::regex("(...)"))) {
    // use the match object here
}

Is this code valid? That is, does the boost::regex object need to live as long as the boost::smatch object, or does that lifetime requirement only apply to the matched string? The code appears to work fine, but I'm worried that I may run into trouble if the smatch object internally keeps a reference to the regular expression used for matching.

I couldn't find this explicitly documented at https://www.boost.org/doc/libs/1_79_0/libs/regex/doc/html/index.html, but it's possible I have missed it.

Answer 1

Firstly, from a design perspective it wouldn't make sense to store a reference to boost::regex in boost::smatch.

Having a look at the implementation confirms that:

template <class BidiIterator, class Allocator>
class match_results
{ 
   // ...

   vector_type            m_subs;                      // subexpressions
   BidiIterator   m_base;                              // where the search started from
   sub_match<BidiIterator> m_null;                     // a null match
   boost::shared_ptr<named_sub_type> m_named_subs;     // Shared copy of named subs in the regex object
   int m_last_closed_paren;                            // Last ) to be seen - used for formatting
   bool m_is_singular;                                 // True if our stored iterators are singular
};