C++ Regex Tutorial: Regular Expressions In C++ With Examples

https://www.softwaretestinghelp.com/regex-in-cpp/

Function Templates Used In C++ Regex
Let’s now discuss some of the important function templates while programming regex in C++.

regex_match()
This function template is used to match the given pattern. This function returns true if the given expression matches the string. Otherwise, the function returns false.

Following is a C++ programming example that demonstrates the regex_match function.

#include <iostream>
#include <string>
#include <regex>
using namespace std;
int main () {
   if (regex_match ("softwareTesting", regex("(soft)(.*)") ))
      cout << "string:literal => matched\n";
   const char mystr[] = "SoftwareTestingHelp";
   string str ("software");
   regex str_expr ("(soft)(.*)");
   if (regex_match (str,str_expr))
      cout << "string:object => matched\n";
   if ( regex_match ( str.begin(), str.end(), str_expr ) )
      cout << "string:range(begin-end)=> matched\n";
   cmatch cm;
   regex_match (mystr,cm,str_expr);
    
   smatch sm;
   regex_match (str,sm,str_expr);
    
   regex_match ( str.cbegin(), str.cend(), sm, str_expr);
   cout << "String:range, size:" << sm.size() << " matches\n";
   
   regex_match ( mystr, cm, str_expr, regex_constants::match_default );
   cout << "the matches are: ";
   for (unsigned i=0; i<sm.size(); ++i) {
      cout << "[" << sm[i] << "] ";
   }
   cout << endl;
   return 0;
}

In the above program, first, we match the string “softwareTesting” against the regular expression “(“(soft)(.*)” using the regex_match function. Subsequently, we also demonstrate different variations of regex_match by passing it a string object, range, etc.

egex_search()
The function regex_search() is used to search for a pattern in the string that matches the regular expression.

Consider the following C++ program that shows the usage of regex_search().

#include <iostream> 
#include <regex> 
#include<string.h> 
using namespace std; 
   
int main() 
{ 
    //string to be searched
    string mystr = "She sells_sea shells in the sea shore"; 
   
    // regex expression for pattern to be searched 
    regex regexp("s[a-z_]+"); 
   
    // flag type for determining the matching behavior (in this case on string objects)
     smatch m; 
   
    // regex_search that searches pattern regexp in the string mystr  
    regex_search(mystr, m, regexp); 
  
    cout<<"String that matches the pattern:"<<endl;
    for (auto x : m) 
        cout << x << " "; 
    return 0; 
}

We specify a string and then a regular expression using the regex object. This string and regex are passed to the regex_search function along with the smatch flag type. The function searches for the first occurrence of pattern in the input string and returns the matched string.

regex_replace()
The function regex_replace() is used to replace the pattern matching to a regular expression with a string.

#include <iostream> 
#include <string> 
#include <regex> 
#include <iterator> 
using namespace std; 
   
int main() 
{  
    string mystr = "This is software testing Help portal \n"; 
     
    cout<<"Input string: "<<mystr<<endl;
       
    // regex to match string beginning with 'p' 
    regex regexp("p[a-zA-z]+"); 
    cout<<"Replace the word 'portal' with word 'website' : "; 
    // regex_replace() for replacing the match with the word 'website'  
    cout << regex_replace(mystr, regexp, "website"); 
     
    string result; 
       
    cout<<"Replace the word 'website' back to 'portal': ";
    // regex_replace( ) for replacing the match back with 'portal' 
    regex_replace(back_inserter(result), mystr.begin(), mystr.end(), 
                  regexp,  "portal"); 
   
    cout << result; 
   
    return 0; 
}

Here, we have an input string. We provide a regular expression to match a string starting with ‘p’. Then we replace the matched word with the word ‘website’. Next, we replace the ‘website’ word back to the portal.