How Many Items Does std::match_results::size() Return in C ?

What Does std::match_results::size() Return?

In C 11, the std::match_results::size() function returns the number of capture groups within a match result object plus one (the whole match value).

Consider the following code:

<code class="cpp">#include <iostream>
#include <string>
#include <regex>

int main() {
    std::string haystack("abcdefabcghiabc");
    std::regex needle("abc");
    std::smatch matches;
    std::regex_search(haystack, matches, needle);
    std::cout <p>This code finds the first occurrence of the substring "abc" in the string "abcdefabcghiabc" and stores the match result in the matches object. Surprisingly, calling matches.size() returns 1 instead of 3 (the expected number of matches).</p>
<p>This behavior is explained by the fact that regex_search() only returns one match and size() includes both the full match and any capture groups. In this case, there are no capture groups, so size() returns 1 (full match only).</p>
<p><strong>Finding Multiple Matches</strong></p>
<p>To find and count multiple matches, use std::sregex_iterator or std::wsregex_iterator (for wide character strings). Here's an example using std::sregex_iterator:</p>
<pre class="brush:php;toolbar:false"><code class="cpp">#include <iostream>
#include <string>
#include <regex>
using namespace std;

int main() {
    std::regex r("abc");
    std::string s = "abcdefabcghiabc";
    int count = 0;
    for (std::sregex_iterator i = std::sregex_iterator(s.begin(), s.end(), r);
        i != std::sregex_iterator();
        ++i) {
        count++;
    }
    cout <p>This code iterates over all matches of "abc" in the string and counts them.</p>
<p><strong>Capture Groups</strong></p>
<p>If your regular expression contains capture groups (parenthesized subexpressions), the size of the match result will include the capture groups as well. For example, if you have a regular expression that matches "abc(def)" and the input string contains "abcdef," the size of the match result will be 2 (full match and capture group "def").</p></regex></string></iostream></code>

The above is the detailed content of How Many Items Does std::match_results::size() Return in C ?. For more information, please follow other related articles on the PHP Chinese website!