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std::regex_token_iterator(3) C++ Standard Libary std::regex_token_iterator(3)


std::regex_token_iterator - std::regex_token_iterator


Defined in header <regex>

class BidirIt,
class CharT = typename std::iterator_traits<BidirIt>::value_type, (since C++11)
class Traits = std::regex_traits<CharT>

> class regex_token_iterator

std::regex_token_iterator is a read-only LegacyForwardIterator that accesses the
individual sub-matches of every match of a regular expression within the underlying
character sequence. It can also be used to access the parts of the sequence that
were not matched by the given regular expression (e.g. as a tokenizer).

On construction, it constructs an std::regex_iterator and on every increment it
steps through the requested sub-matches from the current match_results, incrementing
the underlying regex_iterator when incrementing away from the last submatch.

The default-constructed std::regex_token_iterator is the end-of-sequence iterator.
When a valid std::regex_token_iterator is incremented after reaching the last
submatch of the last match, it becomes equal to the end-of-sequence iterator.
Dereferencing or incrementing it further invokes undefined behavior.

Just before becoming the end-of-sequence iterator, a std::regex_token_iterator may
become a suffix iterator, if the index -1 (non-matched fragment) appears in the list
of the requested submatch indexes. Such iterator, if dereferenced, returns a
match_results corresponding to the sequence of characters between the last match and
the end of sequence.

A typical implementation of std::regex_token_iterator holds the underlying
std::regex_iterator, a container (e.g. std::vector<int>) of the requested submatch
indexes, the internal counter equal to the index of the submatch, a pointer to
std::sub_match, pointing at the current submatch of the current match, and a
std::match_results object containing the last non-matched character sequence (used
in tokenizer mode).

Type requirements

BidirIt must meet the requirements of LegacyBidirectionalIterator.


Several specializations for common character sequence types are defined:

Defined in header <regex>
Type Definition
cregex_token_iterator regex_token_iterator<const char*>
wcregex_token_iterator regex_token_iterator<const wchar_t*>
sregex_token_iterator regex_token_iterator<std::string::const_iterator>
wsregex_token_iterator regex_token_iterator<std::wstring::const_iterator>

Member types

Member type Definition
value_type std::sub_match<BidirIt>
difference_type std::ptrdiff_t
pointer const value_type*
reference const value_type&
iterator_category std::forward_iterator_tag
regex_type basic_regex<CharT, Traits>

Member functions

constructor constructs a new regex_token_iterator
(public member function)
destructor destructs a regex_token_iterator, including the cached value
(implicitly declared) (public member function)
operator= assigns contents
(public member function)
operator== compares two regex_token_iterators
operator!= (public member function)
(removed in C++20)
operator* accesses current submatch
operator-> (public member function)
operator++ advances the iterator to the next submatch
operator++(int) (public member function)


It is the programmer's responsibility to ensure that the std::basic_regex object
passed to the iterator's constructor outlives the iterator. Because the iterator
stores a std::regex_iterator which stores a pointer to the regex, incrementing the
iterator after the regex was destroyed results in undefined behavior.


// Run this code

#include <fstream>
#include <iostream>
#include <algorithm>
#include <iterator>
#include <regex>

int main()
// Tokenization (non-matched fragments)
// Note that regex is matched only two times; when the third value is obtained
// the iterator is a suffix iterator.
const std::string text = "Quick brown fox.";
const std::regex ws_re("\\s+"); // whitespace
std::copy( std::sregex_token_iterator(text.begin(), text.end(), ws_re, -1),
std::ostream_iterator<std::string>(std::cout, "\n"));

std::cout << '\n';

// Iterating the first submatches
const std::string html = R"(<p><a href="">google</a> )"
R"(< a HREF ="">cppreference</a>\n</p>)";
const std::regex url_re(R"!!(<\s*A\s+[^>]*href\s*=\s*"([^"]*)")!!", std::regex::icase);
std::copy( std::sregex_token_iterator(html.begin(), html.end(), url_re, 1),
std::ostream_iterator<std::string>(std::cout, "\n"));