Search and Replace
Transform text with regex.
Search and Replace is a free C++ Academy lesson on CoddyKit. This is lesson 3 of 4. Any 3 lessons of this learning path are free to read in full — after that, CoddyKit PRO unlocks every lesson, plus hands-on practice with a built-in code editor and a 24/7 AI tutor. It is part of the C++ Academy learning path, and your progress syncs across the web and the CoddyKit app. The C++ Academy course includes 4 lessons in total.
regex_replace
std::regex_replace returns a new string where every match of a pattern is replaced by a replacement string.
- The original string is left unchanged.
- By default, all matches are replaced.
#include <iostream>
#include <regex>
int main() {
std::regex p("cat");
std::string s = "the cat and the cat";
std::cout << std::regex_replace(s, p, "dog") << '\n';
return 0;
}Replacing Digits
Patterns make bulk edits easy, such as masking every digit in a string with a placeholder.
#include <iostream>
#include <regex>
int main() {
std::regex p("\\d");
std::string s = "PIN 1234";
std::cout << std::regex_replace(s, p, "*") << '\n';
return 0;
}Backreferences in Replacement
Use $1, $2, ... in the replacement string to insert captured groups, letting you rearrange text.
#include <iostream>
#include <regex>
int main() {
std::regex p("(\\w+) (\\w+)");
std::string s = "John Smith";
std::cout << std::regex_replace(s, p, "$2 $1") << '\n';
return 0;
}The Whole Match: $&
The token $& refers to the entire match, handy for wrapping found text in extra characters.
#include <iostream>
#include <regex>
int main() {
std::regex p("\\d+");
std::string s = "item 42";
std::cout << std::regex_replace(s, p, "[$&]") << '\n';
return 0;
}Replace First Only
Pass std::regex_constants::format_first_only to replace only the first match instead of every occurrence.
#include <iostream>
#include <regex>
int main() {
std::regex p("o");
std::string s = "foo boo";
auto out = std::regex_replace(s, p, "0", std::regex_constants::format_first_only);
std::cout << out << '\n';
return 0;
}Normalizing Whitespace
A common cleanup task: collapse runs of whitespace into a single space.
#include <iostream>
#include <regex>
int main() {
std::regex p("\\s+");
std::string s = "too many spaces";
std::cout << std::regex_replace(s, p, " ") << '\n';
return 0;
}Removing Matches
Replacing with an empty string effectively deletes every match, useful for stripping unwanted characters.
#include <iostream>
#include <regex>
int main() {
std::regex p("[^a-zA-Z]");
std::string s = "a1b2c3";
std::cout << std::regex_replace(s, p, "") << '\n';
return 0;
}Reformatting Dates
Capture parts of a date and reorder them in the replacement to change the format.
#include <iostream>
#include <regex>
int main() {
std::regex p("(\\d{4})-(\\d{2})-(\\d{2})");
std::string s = "2026-05-30";
std::cout << std::regex_replace(s, p, "$3/$2/$1") << '\n';
return 0;
}Keeping Unmatched Text
By default unmatched text is copied through. Use format_no_copy to output only the replacements and drop the rest.
#include <iostream>
#include <regex>
int main() {
std::regex p("\\d+");
std::string s = "a1b2c3";
auto out = std::regex_replace(s, p, "$&", std::regex_constants::format_no_copy);
std::cout << out << '\n';
return 0;
}Censoring Words
Combine alternation with replacement to censor a list of words at once.
#include <iostream>
#include <regex>
int main() {
std::regex p("bad|evil");
std::string s = "a bad and evil idea";
std::cout << std::regex_replace(s, p, "***") << '\n';
return 0;
}Building Up a Result
Because regex_replace returns a string, you can chain transformations by feeding one result into the next call.
#include <iostream>
#include <regex>
int main() {
std::string s = " Hello, World ";
s = std::regex_replace(s, std::regex("\\s+"), " ");
s = std::regex_replace(s, std::regex("^ | $"), "");
std::cout << "[" << s << "]\n";
return 0;
}Quick Check
Test your understanding of replacement syntax.
Recap
You learned to transform text with regex_replace:
- replaces all matches by default and returns a new string
$1,$2,$&insert captures and the whole match- flags like
format_first_onlyandformat_no_copy - chain calls to build complex cleanups
Next, you will learn how to use regex efficiently and avoid common performance traps.
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Frequently Asked Questions
Is the “Search and Replace” lesson free?
Yes — any 3 lessons of the C++ Academy learning path, including “Search and Replace”, are free to read in full here on the web. After that, CoddyKit PRO unlocks every lesson, plus interactive practice with a built-in code editor and a 24/7 AI tutor. The C++ Academy course includes 4 lessons in total.
What will I learn in “Search and Replace”?
Transform text with regex. You practise C++ Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start C++ Academy?
No prior experience is required. C++ Academy on CoddyKit is structured for beginners through advanced learners, so you can start here or from the beginning and move at your own pace. This is lesson 3 of 4.
How long does the “Search and Replace” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this C++ Academy lesson?
Yes. Every C++ Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- regex Basics
- Capturing Groups
- Search and Replace
- Performance Notes