Regular expressions library

< cpp
Defined in header <regex>

The regular expressions library provides a class that represents regular expressions, which are a kind of mini-language used to perform pattern matching within strings. Almost all operations with regexes can be characterized by operating on several of the following objects:

  • Target sequence. The character sequence that is searched for a pattern. This may be a range specified by two iterators, a null-terminated character string or a std::string.
  • Pattern. This is the regular expression itself. It determines what constitutes a match. It is an object of type std::basic_regex, constructed from a string with special syntax. See syntax_option_type for the description of supported syntax variations.
  • Matched array. The information about matches may be retrieved as an object of type std::match_results.
  • Replacement string. This is a string that determines how to replace the matches, see match_flag_type for the description of supported syntax variations.

Main classes

These classes encapsulate a regular expression and the results of matching a regular expression within a target sequence of characters.

regular expression object
(class template)
identifies the sequence of characters matched by a sub-expression
(class template)
identifies one regular expression match, including all sub-expression matches
(class template)


These functions are used to apply the regular expression encapsulated in a regex to a target sequence of characters.

attempts to match a regular expression to an entire character sequence
(function template)
attempts to match a regular expression to any part of a character sequence
(function template)
replaces occurrences of a regular expression with formatted replacement text
(function template)


The regex iterators are used to traverse the entire set of regular expression matches found within a sequence.

iterates through all regex matches within a character sequence
(class template)
iterates through the specified sub-expressions within all regex matches in a given string or through unmatched substrings
(class template)


This class defines the type of objects thrown as exceptions to report errors from the regular expressions library.

reports errors generated by the regular expressions library


The regex traits class is used to encapsulate the localizable aspects of a regex.

provides metainformation about a character type, required by the regex library
(class template)


Defined in namespace std::regex_constants
general options controlling regex behavior
options specific to matching
describes different types of matching errors


#include <iostream>
#include <iterator>
#include <string>
#include <regex>
int main()
    std::string s = "Some people, when confronted with a problem, think "
        "\"I know, I'll use regular expressions.\" "
        "Now they have two problems.";
    std::regex self_regex("REGULAR EXPRESSIONS",
            std::regex_constants::ECMAScript | std::regex_constants::icase);
    if (std::regex_search(s, self_regex)) {
        std::cout << "Text contains the phrase 'regular expressions'\n";
    std::regex word_regex("(\\S+)");
    auto words_begin = 
        std::sregex_iterator(s.begin(), s.end(), word_regex);
    auto words_end = std::sregex_iterator();
    std::cout << "Found "
              << std::distance(words_begin, words_end)
              << " words\n";
    const int N = 6;
    std::cout << "Words longer than " << N << " characters:\n";
    for (std::sregex_iterator i = words_begin; i != words_end; ++i) {
        std::smatch match = *i;
        std::string match_str = match.str();
        if (match_str.size() > N) {
            std::cout << "  " << match_str << '\n';
    std::regex long_word_regex("(\\w{7,})");
    std::string new_s = std::regex_replace(s, long_word_regex, "[$&]");
    std::cout << new_s << '\n';


Text contains the phrase 'regular expressions'
Found 19 words
Words longer than 6 characters:
Some people, when [confronted] with a [problem], think 
"I know, I'll use [regular] [expressions]." Now they have two [problems].