Intro to Regular Expressions (regex.h)

This lesson covers the basics of regular expressions in C using regex.h, so you can check whether a string matches a specific format. It's for anyone searching "C regex.h usage."

A regular expression is a special syntax that describes a string's "shape." You can specify a pattern — like "a phone number is a sequence of digits" — to check whether a string matches it, or extract matching parts. Use functions from the regex.h header.

The example compiles a pattern with regcomp(), and actually performs matching with regexec(). Compared to other languages, this is a distinctly low-level API that requires explicitly separating the compile step from the execution step.

A common early mistake is not realizing a regex object needs to be released once you're done with it. Forgetting to call regfree() causes a memory leak — resource management is a genuinely C-specific concern here.

In real projects, regular expressions are a widely valued skill precisely because, once learned, the same syntax carries over to most other programming languages.

📖 Reference code

#include <stdio.h>
#include <regex.h>

int main(void) {
    regex_t regex;
    regcomp(&regex, "[0-9]+-[0-9]+-[0-9]+", REG_EXTENDED);

    const char *text = "090-1234-5678";
    regmatch_t match;
    if (regexec(&regex, text, 1, &match, 0) == 0) {
        printf("%.*s\n", (int)(match.rm_eo - match.rm_so), text + match.rm_so);
    }

    regex_t regex2;
    regcomp(&regex2, "^[0-9]+$", REG_EXTENDED);
    int result = regexec(&regex2, "12345", 0, NULL, 0);
    printf("%s\n", result == 0 ? "true" : "false");

    regfree(&regex);
    regfree(&regex2);
    return 0;
}

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🧪 This site can't compile or run C directly, so it checks on the spot whether what you typed matches the reference code (scoring happens entirely in your browser — nothing is sent anywhere).