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Introduction to Regular Expressions in C++
Regular expressions, also known nationwide as regex or regexp, are patterns used to match character combinations in strings. In C++, regular expressions can be used with the<regex> library, which is a part of the C++ Standard Library since C++11.
To use the <regex> library, include it in your C++ program:
1 #include <regex>-
Using the
std::regex_matchFunction
The std::regex_match function checks if a given string matches a specified regular expression. If the string matches the regex, std::regex_match returns true, otherwise it returns false.
Here’s an example of how to use std::regex_match:
1 #include <iostream>
2 #include <regex>
3
4 int main() {
5 std::string str = "hello world";
6 std::regex pattern("hello world");
7
8 if (std::regex_match(str, pattern)) {
9 std::cout << "The string matches the regex pattern." << std::endl;
10 } else {
11 std::cout << "The string does not match the regex pattern." << std::endl;
12 }
13
14 return 0;
15}In this example, the output will be “The string matches the regex pattern.” because the string str matches the regex pattern pattern.
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Using the
std::regex_searchFunction
The std::regex_search function searches a given string for a substring that matches a specified regular expression. If a match is found, std::regex_search returns true, otherwise it returns false.
Here’s an example of how to use std::regex_search:
1 #include <iostream>
2#include <regex>
3
4 int main() {
5 std::string str = "hello world";
6 std::regex pattern("world");
7
8 if (std::regex_search(str, pattern)) {
9 std::cout << "A substring that matches the regex pattern was found." << std::endl;
10 } else {
11 std::cout << "No substring that matches the regex pattern was found." << std::endl;
12 }
13
14 return 0;
15}In this example, the output will be “A substring that matches the regex pattern was found.” because the string str contains a substring that matches the regex pattern pattern.
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Regular Expression Syntax
The regular expression syntax used by the <regex> library in C++ is similar to the syntax used by other programming languages and tools.
Follow the below examples of regex patterns and their meanings:
a|b: Matches either the character ‘a’ or the character ‘b’..: Matches any character.^: Matches the start of the string.$: Matches the end of the string.*: Matches zero or more occurrences of the preceding character or pattern.+: Matches one or more occurrences of the preceding character or pattern.?: Matches zero or one occurrence of the preceding character or pattern.{m,n}: Matches at least ‘m’ and at most ‘n’ occurrences of the preceding character or pattern.[a-z]: Matches any lowercase letter.[^a-z]: Matches any character that is not a lowercase letter.\d: Matches any digit.\w: Matches any word character (letters, digits, or underscores).\s: Matches any whitespace character.
By combining these building blocks, you can create complex regular expressions to match specific patterns in strings.
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