⚙️ C++

Expressions régulières en C++ : guide détaillé

Leçon 32 / 39 • Lecture complète de la théorie

What is it and why? (a real-life analogy)

Imagine you are a librarian in a huge library containing millions of books. A reader comes to you and says: "I need all the books with the word 'adventures' in the title, but only those written after 2000, and with a red cover." If you had to look through every book manually, it would take weeks. But if you had a magic magnifying glass that instantly finds books based on such complex descriptions — that is regular expressions.

In the programming world, regular expressions (or regex, short for regular expressions) are a powerful tool for searching and processing text. They allow you to specify not just a specific word, but a pattern that can match many different strings.

Simple analogy: If a regular search is like saying "find me the word 'cat'", then a regular expression is like "find me any three-letter word that starts with 'c' and ends with 't'". The second option will find not only "cat", but also "cot", "cut" (if such a word exists) and any other combinations.

In C++, regular expressions appeared in the C++11 standard and have since become an indispensable tool for:

Data validation — checking if an email, phone number, or zip code is entered correctly
Search and replace — finding all dates in a text and changing their format
Log parsing — extracting needed information from thousands of lines of system records
Lexical analysis — breaking down program code into its constituent parts

Theory: how it works internally

To understand how regular expressions work, you need to imagine a finite automaton. Don't be scared by this term — it's actually quite simple.

Imagine you have a map with cities and roads between them. You start your journey in the city "Start" and move along the roads, reading the letters of the text. Each letter is a signpost indicating which road to take next. If, after reading the entire string, you end up in the city "Finish" — then the string matches the pattern. If you get lost or don't reach the finish — the string does not match.

A regular expression is compiled into such an automaton inside the C++ library. When you pass a string for checking, the regex engine goes through it character by character, trying to find a path from start to finish according to your pattern.

Basic building blocks of regular expressions:

Symbol	Meaning	Example	Explanation
`.`	Any single character	`c.t`	Finds "cat", "cot", "cut"
`*`	0 or more repetitions	`ca*t`	Finds "ct", "cat", "caat", "caaat"
`+`	1 or more repetitions	`ca+t`	Finds "cat", "caat", but not "ct"
`?`	0 or 1 repetition	`ca?t`	Finds "ct" and "cat"
`[]`	Set of characters	`[cb]at`	Finds "cat" and "bat"
`[^]`	Negation of the set	`[^c]at`	Finds "bat", "rat", but not "cat"
`^`	Start of string	`^cat`	Finds "cat" only at the beginning of a string
`$`	End of string	`cat$`	Finds "cat" only at the end of a string
`\|`	Or (alternation)	`cat\|dog`	Finds either "cat" or "dog"
`()`	Grouping	`(ca)+t`	Finds "cat", "cacat", "cacacat"
`d`	Digit (0-9)	`ddd`	Finds any three consecutive digits
`w`	Letter, digit, or underscore	`w+`	Finds a whole word
`s`	Whitespace character	`s+`	Finds one or more spaces

Important: In C++, strings with regular expressions need to be written with double escaping. For example, d in a regular expression becomes "\d" in C++ code. This is because the backslash has a special meaning in C++ strings.

Syntax in C++: all forms and variants

In C++, working with regular expressions is centered in the header file <regex>. The main classes and functions:

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Class/Function	Purpose	Usage Example
`std::regex`	Represents the regular expression itself	`std::regex pattern("\d+");`
`std::regex_match()`	Checks if the entire string matches the pattern	`regex_match("123", pattern)`
`std::regex_search()`	Searches for the first match in a string	`regex_search("abc123def", pattern)`
`std::regex_replace()`	Replaces matches with new text	`regex_replace("abc123", pattern, "X")`