# Regular expressions library

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< 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.

basic_regex (C++11) | regular expression object (class template)
sub_match (C++11) | identifies the sequence of characters matched by a sub-expression (class template)
match_results (C++11) | identifies one regular expression match, including all sub-expression matches (class template)

### Algorithms

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

regex_match (C++11) | attempts to match a regular expression to an entire character sequence (function template)
regex_search (C++11) | attempts to match a regular expression to any part of a character sequence (function template)
regex_replace (C++11) | replaces occurrences of a regular expression with formatted replacement text (function template)

### Iterators

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

regex_iterator (C++11) | iterates through all regex matches within a character sequence (class template)
regex_token_iterator (C++11) | iterates through the specified sub-expressions within all regex matches in a given string or through unmatched substrings (class template)

### Exceptions

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

regex_error (C++11) | reports errors generated by the regular expressions library (class)

### Traits

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

regex_traits (C++11) | provides metainformation about a character type, required by the regex library (class template)

### Constants

Defined in namespace `std::regex_constants`
syntax_option_type (C++11) | general options controlling regex behavior (typedef)
match_flag_type (C++11) | options specific to matching (typedef)
error_type (C++11) | describes different types of matching errors (typedef)

### Example

Run this code

    #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';
    }

Output:

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

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