In C, you can perform pattern matching using regular expressions with the `regex.h` library. Here is an example of how to use this library to match a pattern in a string:

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

int main() {
    char str[100] = "The quick brown fox jumps over the lazy dog.";
    char pattern[20] = "fox";

    regex_t regex;
    int reti;
    reti = regcomp(&regex, pattern, 0); // Compile the regular expression
    if (reti) {
        fprintf(stderr, "Could not compile regex\n");
        return 1;
    }
    
    reti = regexec(&regex, str, 0, NULL, 0); // Execute the regular expression
    if (!reti) {
        printf("Match found\n");
    } else if (reti == REG_NOMATCH) {
        printf("No match found\n");
    } else {
        fprintf(stderr, "Regex match failed\n");
        return 1;
    }

    regfree(&regex); // Free the regular expression structure

    return 0;
}
```

In this example, we have a string `str` and a pattern `pattern` that we want to match in the string. We first create a `regex_t` object to store the compiled regular expression. We then use the `regcomp()` function to compile the regular expression from the `pattern` string.

Next, we use the `regexec()` function to execute the regular expression against the `str` string. If the regular expression matches the string, `regexec()` returns 0, and we print a message indicating that a match was found. If the regular expression does not match the string, `regexec()` returns `REG_NOMATCH`, and we print a message indicating that no match was found.

Finally, we free the `regex_t` object using the `regfree()` function.

Note that regular expressions can be complex, and this example only scratches the surface of what is possible with the `regex.h` library. For more information, consult the C documentation and regular expression tutorials.

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