std::regex_match(3)

Okay bestie, this is std::regex_match(3), live from the C++ Standard Libary (their spelling, not mine 💀), and it’s giving “does this regex match the whole string” main character energy 👑.

🌸 NAME 🌸

std::regex_match — std::regex_match (yes, that’s literally the name, no notes 💅).

✨ Synopsis ✨

Okay so here’s the whole overload family, and yes every (since C++11) label stays exactly where it was, no cap:

          Defined in header <regex>
          template< class BidirIt,

                    class Alloc, class CharT, class Traits >
          bool regex_match( BidirIt first, BidirIt last,
                            std::match_results<BidirIt, Alloc>& m,
       (1) (since C++11)
                            const std::basic_regex<CharT, Traits>& e,
                            std::regex_constants::match_flag_type flags =

                                std::regex_constants::match_default );
          template< class BidirIt,

                    class CharT, class Traits >
          bool regex_match( BidirIt first, BidirIt last,
       (2) (since C++11)
                            const std::basic_regex<CharT, Traits>& e,
                            std::regex_constants::match_flag_type flags =

                                std::regex_constants::match_default );
          template< class CharT, class Alloc, class Traits >

          bool regex_match( const CharT* str,
                            std::match_results<const CharT*, Alloc>& m,
       (3) (since C++11)
                            const std::basic_regex<CharT, Traits>& e,
                            std::regex_constants::match_flag_type flags =

                                std::regex_constants::match_default );
          template< class STraits, class SAlloc,

                    class Alloc, class CharT, class Traits >
          bool regex_match( const std::basic_string<CharT, STraits, SAlloc>&
          s,
                            std::match_results<
                                typename std::basic_string<CharT, STraits,
       (4) (since C++11)
          SAlloc>::
                                    const_iterator,
                                Alloc>& m,
                            const std::basic_regex<CharT, Traits>& e,
                            std::regex_constants::match_flag_type flags =

                                std::regex_constants::match_default );
          template< class CharT, class Traits >

          bool regex_match( const CharT* str,
                            const std::basic_regex<CharT, Traits>& e,
       (5) (since C++11)
                            std::regex_constants::match_flag_type flags =

                                std::regex_constants::match_default );
          template< class STraits, class SAlloc,

                    class CharT, class Traits >
          bool regex_match( const std::basic_string<CharT, STraits, SAlloc>&
          s,
       (6) (since C++11)
                            const std::basic_regex<CharT, Traits>& e,
                            std::regex_constants::match_flag_type flags =

                                std::regex_constants::match_default );
          template< class STraits, class SAlloc,

                    class Alloc, class CharT, class Traits >
          bool regex_match( const std::basic_string<CharT, STraits,
          SAlloc>&&,
                            std::match_results<
                                typename std::basic_string<CharT, STraits,
          SAlloc>::
       (7) (since C++11)
                                    const_iterator,
                                Alloc>&,
                            const std::basic_regex<CharT, Traits>&,
                            std::regex_constants::match_flag_type flags =

                                std::regex_constants::match_default ) =
          delete;

Determines if the regular expression e matches the entire target character sequence, which may be specified as std::string, a C-string, or an iterator pair 💖.

1. Determines if there is a match between the regular expression e and the entire target character sequence [first, last), taking into account the effect of flags. When determining if there is a match, only potential matches that match the entire character sequence are considered. Match results are returned in m.
2. Behaves as (1) above, omitting the match results.
3. Returns std::regex_match(str, str + std::char_traits<CharT>::length(str), m, e, flags).
4. Returns std::regex_match(s.begin(), s.end(), m, e, flags).
5. Returns std::regex_match(str, str + std::char_traits<CharT>::length(str), e, flags).
6. Returns std::regex_match(s.begin(), s.end(), e, flags).
7. The overload (4) is prohibited from accepting temporary strings, otherwise this function populates match_results m with string iterators that become invalid immediately.

Note that regex_match will only successfully match a regular expression to an entire character sequence, whereas std::regex_search will successfully match subsequences. Bestie, that distinction is important 😭.

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💖 Parameters 💖

• first, last — the target character range to apply the regex to, given as iterators
• m — the match results
• str — the target string, given as a null-terminated C-style string
• s — the target string, given as a std::basic_string
• e — the regular expression
• flags — flags used to determine how the match will be performed

🧚 Type requirements 🧚

• BidirIt must meet the requirements of LegacyBidirectionalIterator.

🔥 Return value 🔥

Returns true if a match exists, false otherwise. In either case, the object m is updated, as follows:

If the match does not exist:

• m.ready() == true
• m.empty() == true
• m.size() == 0

If the match exists:

• m.ready() — true
• m.empty() — false
• m.size() — number of marked subexpressions plus 1, that is, 1 + e.mark_count()
• m.prefix().first — first
• m.prefix().second — first
• m.prefix().matched — false (the match prefix is empty)
• m.suffix().first — last
• m.suffix().second — last
• m.suffix().matched — false (the match suffix is empty)
• m[0].first — first
• m[0].second — last
• m[0].matched — true (the entire sequence is matched)
• m[n].first — the start of the sequence that matched marked sub-expression n, or last if the subexpression did not participate in the match
• m[n].second — the end of the sequence that matched marked sub-expression n, or last if the subexpression did not participate in the match
• m[n].matched — true if sub-expression n participated in the match, false otherwise

🦋 Notes 🦋

Because regex_match only considers full matches, the same regex may give different matches between regex_match and std::regex_search:

std::regex re("Get|GetValue");
std::cmatch m;
std::regex_search("GetValue", m, re);  // returns true, and m[0] contains "Get"
std::regex_match ("GetValue", m, re);  // returns true, and m[0] contains "GetValue"
std::regex_search("GetValues", m, re); // returns true, and m[0] contains "Get"
std::regex_match ("GetValues", m, re); // returns false

🎀 Example 🎀

// Run this code

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

int main()
{
    // Simple regular expression matching
    const std::string fnames[] = {"foo.txt", "bar.txt", "baz.dat", "zoidberg"};
    const std::regex txt_regex("[a-z]+\\.txt");

    for (const auto& fname : fnames)
        std::cout << fname << ": " << std::regex_match(fname, txt_regex) << '\n';

    // Extraction of a sub-match
    const std::regex base_regex("([a-z]+)\\.txt");
    std::smatch base_match;

    for (const auto& fname : fnames)
        if (std::regex_match(fname, base_match, base_regex))
            // The first sub_match is the whole string; the next
            // sub_match is the first parenthesized expression.
            if (base_match.size() == 2)
            {
                std::ssub_match base_sub_match = base_match[1];
                std::string base = base_sub_match.str();
                std::cout << fname << " has a base of " << base << '\n';
            }

    // Extraction of several sub-matches
    const std::regex pieces_regex("([a-z]+)\\.([a-z]+)");
    std::smatch pieces_match;

    for (const auto& fname : fnames)
        if (std::regex_match(fname, pieces_match, pieces_regex))
        {
            std::cout << fname << '\n';
            for (std::size_t i = 0; i < pieces_match.size(); ++i)
            {
                std::ssub_match sub_match = pieces_match[i];
                std::string piece = sub_match.str();
                std::cout << "  submatch " << i << ": " << piece << '\n';
            }
        }
}

🍓 Output 🍓

foo.txt: 1
bar.txt: 1
baz.dat: 0
zoidberg: 0
foo.txt has a base of foo
bar.txt has a base of bar
foo.txt
  submatch 0: foo.txt
  submatch 1: foo
  submatch 2: txt
bar.txt
  submatch 0: bar.txt
  submatch 1: bar
  submatch 2: txt
baz.dat
  submatch 0: baz.dat
  submatch 1: baz
  submatch 2: dat

💀 Defect reports 💀

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

• DR: LWG 2329
• Applied to: C++11
• Behavior as published: basic_string rvalues were accepted, which was likely to result in dangling iterators
• Correct behavior: rejected via a deleted overload

🌈 See also 🌈

• basic_regex (C++11) — regular expression object (class template)
• match_results (C++11) — identifies one regular expression match, including all sub-expression matches (class template)
• regex_search (C++11) — attempts to match a regular expression to any part of a character sequence (function template)

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xoxo, this cutie came from http://cppreference.com, version/date 2024.06.10, page std::regex_match(3) 💖✨