std::regex_search(3)

💖 std::regex_search 💖

okay bestie, welcome to the std::regex_search(3) page from the C++ Standard Libary — section 3, the manual-page girlies. This is the function that asks: does my regex e match some subsequence somewhere in this target character sequence? Not the whole thing — that's a different function's job, and we will spill that tea below. 💅

🌸 NAME 🌸

• std::regex_search — std::regex_search, bestie. That's the name. Iconic.

✨ Synopsis ✨

Defined in header <regex>

(1) (since C++11)

template< class BidirIt,
          class Alloc, class CharT, class Traits >
bool regex_search( BidirIt first, BidirIt last,
                   std::match_results<BidirIt,Alloc>& m,
                   const std::basic_regex<CharT,Traits>& e,
                   std::regex_constants::match_flag_type flags =
                       std::regex_constants::match_default );

(2) (since C++11)

template< class CharT, class Alloc, class Traits >
bool regex_search( const CharT* str,
                   std::match_results<const CharT*,Alloc>& m,
                   const std::basic_regex<CharT,Traits>& e,
                   std::regex_constants::match_flag_type flags =
                       std::regex_constants::match_default );

(3) (since C++11)

template< class STraits, class SAlloc,
          class Alloc, class CharT, class Traits >
bool regex_search( const std::basic_string<CharT,STraits,SAlloc>& s,
                   std::match_results<
                       typename
                       std::basic_string<CharT,STraits,SAlloc>::const_iterator,
                       Alloc
                   >& m,
                   const std::basic_regex<CharT, Traits>& e,
                   std::regex_constants::match_flag_type flags =
                       std::regex_constants::match_default );

(4) (since C++11)

template< class BidirIt,
          class CharT, class Traits >
bool regex_search( BidirIt first, BidirIt last,
                   const std::basic_regex<CharT,Traits>& e,
                   std::regex_constants::match_flag_type flags =
                       std::regex_constants::match_default );

(5) (since C++11)

template< class CharT, class Traits >
bool regex_search( const CharT* str,
                   const std::basic_regex<CharT,Traits>& e,
                   std::regex_constants::match_flag_type flags =
                       std::regex_constants::match_default );

(6) (since C++11)

template< class STraits, class SAlloc,
          class CharT, class Traits >
bool regex_search( const std::basic_string<CharT,STraits,SAlloc>& s,
                   const std::basic_regex<CharT,Traits>& e,
                   std::regex_constants::match_flag_type flags =
                       std::regex_constants::match_default );

(7) (since C++11)

template< class STraits, class SAlloc,
          class Alloc, class CharT, class Traits >
bool regex_search( const std::basic_string<CharT,STraits,SAlloc>&&,
                   std::match_results<
                       typename
                       std::basic_string<CharT,STraits,SAlloc>::const_iterator,
                       Alloc
                   >&,
                   const std::basic_regex<CharT, Traits>&,
                   std::regex_constants::match_flag_type flags =
                       std::regex_constants::match_default ) =
    delete;

So here's the whole point: this function determines if there is a match between the regular expression e and some subsequence in the target character sequence. Yes, some subsequence — she's flexible like that. 🦋

• 1) Analyzes generic range [first, last). Match results are returned in m.
• 2) Analyzes a null-terminated string pointed to by str. Match results are returned in m.
• 3) Analyzes a string s. Match results are returned in m.
• 4-6) Equivalent to (1-3), just omits the match results.
• 7) The overload (3) is prohibited from accepting temporary strings, otherwise this function populates match_results m with string iterators that become invalid immediately.

regex_search will successfully match any subsequence of the given sequence, whereas std::regex_match will only return true if the regular expression matches the entire sequence. So if you're looking for a whole-string situation, wrong function, bestie. 💅

🫶 Parameters 🫶

• first, last — a range identifying the target character sequence.
• str — a pointer to a null-terminated target character sequence.
• s — a string identifying target character sequence.
• e — the std::regex that should be applied to the target character sequence.
• m — the match results.
• flags — std::regex_constants::match_flag_type governing search behavior.

🎀 Type requirements 🎀

• BidirIt must meet the requirements of LegacyBidirectionalIterator.
• Alloc must meet the requirements of Allocator.

🌈 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 — m[0].first
• m.prefix().matched — m.prefix().first != m.prefix().second
• m.suffix().first — m[0].second
• m.suffix().second — last
• m.suffix().matched — m.suffix().first != m.suffix().second
• m[0].first — the start of the matching sequence
• m[0].second — the end of the matching sequence
• m[0].matched — true
• m[n].first — the start of the sequence that matched marked subexpression n, or last if the subexpression did not participate in the match
• m[n].second — the end of the sequence that matched marked subexpression 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 🧚

In order to examine all matches within the target sequence, std::regex_search may be called in a loop, restarting each time from m[0].second of the previous call. std::regex_iterator offers an easy interface to this iteration. So if you're about to write a whole loop yourself, bestie, maybe peek at std::regex_iterator first. 💅

🔥 Example 🔥

// Run this code

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

int main()
{
    std::string lines[] = {"Roses are #ff0000",
                           "violets are #0000ff",
                           "all of my base are belong to you"};

    std::regex color_regex("#([a-f0-9]{2})"
                            "([a-f0-9]{2})"
                            "([a-f0-9]{2})");

    // simple match
    for (const auto& line : lines)
        std::cout << line << ": " << std::boolalpha
                  << std::regex_search(line, color_regex) << '\n';
    std::cout << '\n';

    // show contents of marked subexpressions within each match
    std::smatch color_match;
    for (const auto& line : lines)
        if (std::regex_search(line, color_match, color_regex))
        {
            std::cout << "matches for '" << line << "'\n";
            std::cout << "Prefix: '" << color_match.prefix() << "'\n";
            for (std::size_t i = 0; i < color_match.size(); ++i)
                std::cout << i << ": " << color_match[i] << '\n';
            std::cout << "Suffix: '" << color_match.suffix() << "\'\n\n";
        }

    // repeated search (see also std::regex_iterator)
    std::string log(R"(
        Speed:  366
        Mass:   35
        Speed:  378
        Mass:   32
        Speed:  400
        Mass:   30)");
    std::regex r(R"(Speed:\t\d*)");
    for (std::smatch sm; regex_search(log, sm, r);)
    {
        std::cout << sm.str() << '\n';
        log = sm.suffix();
    }

    // C-style string demo
    std::cmatch cm;
    if (std::regex_search("this is a test", cm, std::regex("test")))
        std::cout << "\nFound " << cm[0] << " at position "
                  << cm.prefix().length() << '\n';
}

💀 Output 💀

Roses are #ff0000: true
violets are #0000ff: true
all of my base are belong to you: false

matches for 'Roses are #ff0000'
Prefix: 'Roses are '
0: #ff0000
1: ff
2: 00
3: 00
Suffix: ''

matches for 'violets are #0000ff'
Prefix: 'violets are '
0: #0000ff
1: 00
2: 00
3: ff
Suffix: ''

Speed:  366
Speed:  378
Speed:  400

Found test at position 10

🧚 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 — regular expression object (C++11) (class template)
• match_results — identifies one regular expression match, including all sub-expression matches (C++11) (class template)
• regex_match — attempts to match a regular expression to an entire character sequence (C++11) (function template)

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Source/version: http://cppreference.com, dated 2024.06.10, page std::regex_search(3). That's the whole manual page, bestie — go match some subsequences and slay. 💖✨🎀