How does `std::match_results::size()` determine the number of matches in a regular expression?

Determining the Size of Regex Matches with std::match_results::size()

std::match_results::size() provides insights into the number of match groups and the full match value within a regex match. Let's delve into its functionality.

In C 11 code, std::match_results::size() is used to obtain information about regex matches. It returns a count that includes the entire match plus any capture groups that may have been specified in the regular expression. However, the value it provides may differ from the expected number of matches.

Consider the following example:

<code class="cpp">std::string haystack("abcdefabcghiabc");
std::regex needle("abc");
std::smatch matches;
std::regex_search(haystack, matches, needle);
std::cout << matches.size() << std::endl;</code>

In this case, you might anticipate getting a result of 3 matches. However, you will only receive 1 because regex_search() retrieves merely one match. The size() function reports the count of capture groups plus the entire match.

To clarify, std::match_results holds match details and any discovered submatches. If a regex search succeeds, it will not be empty. It contains a series of sub_match objects, with the initial one representing the full match. Other sub_match objects in the match_results object correspond to subgroups within the regex expression (indicated by parentheses).

For instance, consider the following regular expression:

<code class="cpp">"ab(c)"</code>

It has a capture group that traps the character "c." The following code employs std::regex_iterator to find multiple matches, safeguarding against string destruction:

<code class="cpp">std::string str("abcdefabcghiabc");
std::regex rgx1("abc");
smatch smtch;
while (regex_search(str, smtch, rgx1)) {
    std::cout << smtch[0] << std::endl;
    str = smtch.suffix().str();
}</code>

This code prints "abc" three times.

Alternatively, std::sregex_iterator can be utilized, providing a more efficient and string-preserving method:

<code class="cpp">std::string s = "abcdefabcghiabc";
for (std::sregex_iterator i = std::sregex_iterator(s.begin(), s.end(), r);
     i != std::sregex_iterator();
     ++i)
{
    std::smatch m = *i;
    std::cout << "Match value: " << m.str() << "\n";
    std::cout << "    Capture: " << m[1].str() << "\n";
}</code>

This code prints the full match and capture values, offering a thorough understanding of std::match_results::size() behavior.

The above is the detailed content of How does `std::match_results::size()` determine the number of matches in a regular expression?. For more information, please follow other related articles on the PHP Chinese website!