Chapter 06 Regular Expression

6.1 Introduction

Regular expressions are not part of the C++ language and therefore we only briefly introduced it here.

Regular expressions describe a pattern of string matching. The general use of regular expressions is mainly to achieve the following three requirements:

1. Check if a string contains some form of substring;
2. Replace the matching substrings;
3. Take the eligible substring from a string.

Regular expressions are text patterns consisting of ordinary characters (such as a to z) and special characters. A pattern describes one or more strings to match when searching for text. Regular expressions act as a template to match a character pattern to the string being searched.

Ordinary characters

Normal characters include all printable and unprintable characters that are not explicitly specified as metacharacters. This includes all uppercase and lowercase letters, all numbers, all punctuation, and some other symbols.

Special characters

A special character is a character with special meaning in a regular expression and is also the core matching syntax of a regular expression. See the table below:

Quantifiers

The qualifier is used to specify how many times a given component of a regular expression must appear to satisfy the match. See the table below:

With these two tables, we can usually read almost all regular expressions.

6.2 std::regex and Its Related

The most common way to match string content is to use regular expressions. Unfortunately, in traditional C++, regular expressions have not been supported by the language level, and are not included in the standard library. C++ is a high-performance language. In the development of background services, the use of regular expressions is also used when judging URL resource links. The most mature and common practice in the industry.

The general solution is to use the regular expression library of boost. C++11 officially incorporates the processing of regular expressions into the standard library, providing standard support from the language level and no longer relying on third parties.

The regular expression library provided by C++11 operates on the std::string object, and the pattern std::regex (essentially std::basic_regex) is initialized and matched by std::regex_match Produces std::smatch (essentially the std::match_results object).

We use a simple example to briefly introduce the use of this library. Consider the following regular expression:

• [az]+\.txt: In this regular expression, [az] means matching a lowercase letter, + can match the previous expression multiple times, so [az]+ can Matches a string of lowercase letters. In the regular expression, a . means to match any character, and \. means to match the character ., and the last txt means to match txt exactly three letters. So the content of this regular expression to match is a text file consisting of pure lowercase letters.

std::regex_match is used to match strings and regular expressions, and there are many different overloaded forms. The simplest form is to pass std::string and a std::regex to match. When the match is successful, it will return true, otherwise, it will return false. For example:

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

int main() {
    std::string fnames[] = {"foo.txt", "bar.txt", "test", "a0.txt", "AAA.txt"};
    // In C++, `\` will be used as an escape character in the string.
    // In order for `\.` to be passed as a regular expression,
    // it is necessary to perform second escaping of `\`, thus we have `\\.`
    std::regex txt_regex("[a-z]+\\.txt");
    for (const auto &fname: fnames)
        std::cout << fname << ": " << std::regex_match(fname, txt_regex) << std::endl;
}

Another common form is to pass in the three arguments std::string/std::smatch/std::regex. The essence of std::smatch is actually std::match_results. In the standard library, std::smatch is defined as std::match_results<std::string::const_iterator>, which means match_results of a substring iterator type. Use std::smatch to easily get the matching results, for example:

std::regex base_regex("([a-z]+)\\.txt");
std::smatch base_match;
for(const auto &fname: fnames) {
    if (std::regex_match(fname, base_match, base_regex)) {
        // the first element of std::smatch matches the entire string
        // the second element of std::smatch matches the first expression
        // with brackets
        if (base_match.size() == 2) {
            std::string base = base_match[1].str();
            std::cout << "sub-match[0]: " << base_match[0].str() << std::endl;
            std::cout << fname << " sub-match[1]: " << base << std::endl;
        }
    }
}

The output of the above two code snippets is:

foo.txt: 1
bar.txt: 1
test: 0
a0.txt: 0
AAA.txt: 0
sub-match[0]: foo.txt
foo.txt sub-match[1]: foo
sub-match[0]: bar.txt
bar.txt sub-match[1]: bar

Conclusion

This section briefly introduces the regular expression itself, and then introduces the use of the regular expression library through a practical example based on the main requirements of using regular expressions.

Exercise

In web server development, we usually want to serve some routes that satisfy a certain condition. Regular expressions are one of the tools to accomplish this. Given the following request structure:

struct Request {
    // request method, POST, GET; path; HTTP version
    std::string method, path, http_version;
    // use smart pointer for reference counting of content
    std::shared_ptr<std::istream> content;
    // hash container, key-value dict
    std::unordered_map<std::string, std::string> header;
    // use regular expression for path match
    std::smatch path_match;
};

Requested resource type:

typedef std::map<
    std::string, std::unordered_map<
        std::string,std::function<void(std::ostream&, Request&)>>> resource_type;

And server template:

template <typename socket_type>
class ServerBase {
public:
    resource_type resource;
    resource_type default_resource;

    void start() {
        // TODO
    }
protected:
    Request parse_request(std::istream& stream) const {
        // TODO
    }
}

Please implement the member functions start() and parse_request. Enable server template users to specify routes as follows:

template<typename SERVER_TYPE>
void start_server(SERVER_TYPE &server) {

    // process GET request for /match/[digit+numbers],
    // e.g. GET request is /match/abc123, will return abc123
    server.resource["fill_your_reg_ex"]["GET"] =
        [](ostream& response, Request& request)
    {
        string number=request.path_match[1];
        response << "HTTP/1.1 200 OK\r\nContent-Length: " << number.length()
            << "\r\n\r\n" << number;
    };

    // peocess default GET request;
    // anonymous function will be called
    // if no other matches response files in folder web/
    // default: index.html
    server.default_resource["fill_your_reg_ex"]["GET"] =
        [](ostream& response, Request& request)
    {
        string filename = "www/";

        string path = request.path_match[1];

        // forbidden use `..` access content outside folder web/
        size_t last_pos = path.rfind(".");
        size_t current_pos = 0;
        size_t pos;
        while((pos=path.find('.', current_pos)) != string::npos && pos != last_pos) {
            current_pos = pos;
            path.erase(pos, 1);
            last_pos--;
        }

        // (...)
    };

    server.start();
}

An suggested solution can be found here.

Further Readings

1. Comments from std::regex's author
2. Library document of Regular Expression