C++ Tutorial 21
In this part of my C++ tutorial I finish covering C++ with Regular Expressions. If you missed the 1st part watch it first.
This time I’ll cover Back References, Substitutions, Look Ahead, Or Conditionals and 3 problems for you to solve. All of the heavily commented code follows the video below. Use it to receive the best results in your studies.
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Code from the Video
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#include <cstdlib> #include <iostream> #include <string> #include <vector> #include <ctime> #include <numeric> #include <cmath> #include <sstream> #include <thread> #include <ctime> #include <regex> void PrintMatches(std::string str, std::regex reg){ // Used when your searching a string std::smatch matches; // Determines if there is a match and match // results are returned in matches while(std::regex_search(str, matches, reg)){ // Get the first match std::cout << matches.str(1) << "\n"; // Eliminate the previous match and create // a new string to search str = matches.suffix().str(); } std::cout << "\n"; } // [ ] : Match what is in the brackets // [^ ] : Match anything not in the brackets // ( ) : Return surrounded submatch // . : Match any 1 character or space // + : Match 1 or more of what proceeds // ? : Match 0 or 1 // * : Match 0 or More // *? : Lazy match the smallest match // \b : Word boundary // ^ : Beginning of String // $ : End of String // \n : Newline // \d : Any 1 number // \D : Anything but a number // \w : Same as [a-zA-Z0-9_] // \W : Same as [^a-zA-Z0-9_] // \s : Same as [\f\n\r\t\v] // \S : Same as [^\f\n\r\t\v] // {5} : Match 5 of what proceeds the curly brackets // {5,7} : Match values that are between 5 and 7 in length int main() { // ---------- Back References ---------- // A back reference allows you to to reuse the expression // that proceeds it // Grab a double word std::string str1 = "The cat cat fell out the window"; // Match word boundary, 1 or more characters, space // and the same word that proceeded it // \\1 is used to refer to the 1st match surrounded // by () std::regex reg1 ("(\\b\\w+)\\s+\\1"); PrintMatches(str1,reg1); // ---------- Back Reference Substitutions ---------- // Replace the bold tags in the link with no tags std::string str2 = "<a href='#'><b>The Link</b></a>"; // Regex matches bold tags and grabs the text between // them to be used by the back reference std::regex reg2 ("<b>(.*?)</b>"); // Holds the new string std::string result; // regex_replace makes a copy of the original // along with the replaced parts // back_inserter inserts the final results // into result // $1 references the subexpression surrounded // with () ex. (.*?) std::regex_replace(std::back_inserter(result), str2.begin(), str2.end(), reg2, "$1"); std::cout << result << "\n"; // ---------- Another Back Reference Substitution ---------- // Here I want to surround all phone number // area codes with () std::string str3 = "412-555-1212"; // I create 2 subexpressions std::regex reg3 ("([\\d]{3})-([\\d]{3}-[\\d]{4})"); // Holds the new string std::string res3; // The output is the area code surrounded with () // and then the 2nd subexpression std::regex_replace(std::back_inserter(res3), str3.begin(), str3.end(), reg3, "($1)$2"); std::cout << res3 << "\n"; // ---------- PROBLEM ---------- // Receive a string like this std::string str4 = "https://www.youtube.com " "http://www.google.com"; // Grab the URL and then provide the following output // using a back reference substitution // <a href='https://www.youtube.com'>www.youtube.com</a> // <a href='https://www.google.com'>www.google.com</a> // Grab the whole URL and the web page name std::regex reg4 ("(https?://([\\w.]+))"); // Holds the new string std::string res4; std::regex_replace(std::back_inserter(res4), str4.begin(), str4.end(), reg4, "<a href='$1'>$2</a>\n"); std::cout << res4 << "\n"; // ---------- Look Ahead ---------- // A look ahead defines a pattern to match but not // return // You define the expression to look for but not // return // like this (?=expression) std::string str5 = " one two three four "; // I don't want to return the word boundaries std::regex reg5 ("(\\w+(?=\\b))"); PrintMatches(str5,reg5); // ---------- OR Conditional ---------- // You can use | to define the matches you'll except std::string str6 = "1. Dog 2. Cat 3. Turtle"; std::regex reg6 ("\\d\\.\\s(Dog|Cat)"); PrintMatches(str6,reg6); // ---------- PROBLEM ---------- // Create a regex that will match for 5 digit zip // codes or zip codes with 5 digits a dash and // then 4 digits std::string str7 = "12345 12345-1234 1234 12346-333"; std::regex reg7 ("(\\d{5}-\\d{4}|\\d{5}\\s)"); PrintMatches(str7,reg7); // ---------- PROBLEM ---------- // Find all of the following phone numbers // and then print them std::string str8 = "14125551212 4125551212 " "(412)5551212 412 555 1212 412-555-1212 " "1-412-555-1212"; std::regex reg8 ("((1?)(-| ?)(\\()?(\\d{3})(\\)|-| |\\)-|\\) )?(\\d{3})(-| )?(\\d{4}|\\d{4}))"); PrintMatches(str8,reg8); return 0; } |
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