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 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.


[edit] 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)

[edit] Algorithms

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)

[edit] Iterators

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)

[edit] Exceptions

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

[edit] Traits

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)

[edit] Constants

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

[edit] Example

#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].