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This chapter is from the book

This chapter is from the book

20.4. Matching and Searching Functions

This section summarizes the syntax for the matching and searching functions. Don’t forget to include the <regex> header. Also, remember that regex symbols are part of the std namespace, so either include a using statement or identify each name with its std prefix.

#include <regex>
using std::regex;

Next, you need to specify a regular-expression pattern by creating a regex object:

regex  name(pattern_string [,flags])

This regex constructor includes an optional flags argument. The most useful flag is regex_constants::icase, which turns off case sensitivity.

regex objects are unusual in that they are compiled and built at runtime, not compile time. Consequently, you want to create as few regex objects as possible, because creating many such objects impacts runtime performance. You should make your regex objects global variables, or pass them by reference if you need to.

After constructing a regex object, you can perform matching and searching by calling the regex_match and regex_search functions. Each of these returns a Boolean value. The regex_match function returns true if the target_string matches the pattern stored in regex_obj exactly: That is, the entire target_string must match the pattern completely.

regex_match(target_string, regex_obj)

The regex_search function returns true if the target string contains any substring that matches the pattern stored in regex_obj.

regex_search(target_string, regex_obj)

For example, consider the task of finding a repeated word in the sentence:

The the cow jumped over the the moon.

The following statements execute this search:

#include <regex>
#include <string>
#include <iostream>
using std::string;
using std::cout;
using std::endl;
. . .
std::regex reg1("([A-Za-z]+) \\1");
string target = "The the cow jumped over the the moon.";
if (std::regex_search(target, reg1)) {
     cout << "A repeated word was found." << endl;
}

The call to regex_search failed to find the first repeated-word occurrence (“The the”) because the first word was capitalized and the second was not. That did not matter in this instance, because there was another repeated-word sequence (“the the”). However, sometimes you are interested in getting the position of the first match, so it can matter.

To find the position of the first substring found, it’s necessary to include another argument:

regex_search(target_string, match_info, regex_obj)

The match_info argument is an object of type cmatch, smatch, or wmatch, corresponding to the following formats: C-string, string object, and wide-character string. The format must match the type of target_string; in this case, the string class is used for the target string, so match_info must have type smatch.

For example:

std::smatch sm;
std::regex reg1("([A-Za-z]+) \\1");
std::string target="The the cow jumped over the the moon.";
bool b = std::regex_search(target, sm, reg1);

The match_info object (sm in this case) can be used to obtain information about the search, as follows:

match_obj.str()             // Return the matched string
match_obj.position()    // Return the position of the matched string

For example, the following call finds the position of the first repeated word:

std::smatch sm;
std::regex reg1("([A-Za-z]+) \\1");
std::string target="The the cow jumped over the the moon.";
bool b = std::regex_search(target, sm, reg1);
cout << "Match found at pos. " << sm.position() << endl;
cout << "Pattern found was: " << sm.str() << endl;

These statements print:

Match was found at pos. 24
Pattern found was: the the

Turning off case sensitivity causes a match to be found at position 0 instead.

std::regex reg1("([A-Za-z]+) \\1", regex_constants::icase);

You can also obtain information about groups—patterns enclosed in parentheses—within the matched string. This information only applies to groups within the first substring found; regex_search finds only one substring and then it stops searching. (The next section describes how to find multiple substrings.)

match_obj.str(n)        // Return nth group within the matched substring
match_obj.position(n)   // Return position of nth group

For example, you can use match_object (sm in this case) to get information on the group found within the first matched substring.

cout << "Text of sub-group: " << sm.str(1);

Assuming case sensitivity is turned off, this prints:

Text of sub-group: The
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