# std::match_results<BidirIt,Alloc>::prefix

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< cpp‎ | regex‎ | match results

const_reference prefix() const; |  | (since C++11)

Obtains a reference to the std::sub_match object representing the target sequence between the start of the beginning of the target sequence and the start of the entire match of the regular expression.

`ready()` must be true. Otherwise, the behavior is undefined.

### Return value

Reference to the unmatched prefix.

### Example

Run this code

    #include <iostream>
    #include <regex>
    #include <string>

    int main()
    {
        std::regex re("a(a)*b");
        std::string target("baaaby");
        std::smatch sm;

        std::regex_search(target, sm, re);
        std::cout << sm.prefix().str() << '\n';
    }

Output:

    b

### See also

suffix | returns sub-sequence between the end of the full match and the end of the target sequence (public member function)

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