How to Replace a Substring in a String using Regular Expression in C - Step by Step Examples

How to Replace a Substring in a String using Regular Expression in C ?

Answer

To replace a substring in a string using regular expressions in C, you can use the POSIX regular expression library. This involves compiling a regular expression, searching for matches, and performing the replacement.

✐ Examples

1 Replace Substring Matching a Regular Expression in a String

In this example,

We include the necessary headers regex.h for regular expression functions and string.h for string handling functions.
We create a function named replaceRegex to replace a substring matching a regular expression.
Inside the function, we compile the regular expression using regcomp.
We use regexec to find the substring that matches the pattern.
If a match is found, we copy the part before the match, add the new substring, and then add the part after the match to the result.
We return the modified string.
We call the replaceRegex function with the original string, the regex pattern to be replaced, and the new substring.
Finally, we print the modified string to the standard output.

C Program

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

void replaceRegex(char *str, const char *pattern, const char *replacement) {
    regex_t regex;
    regmatch_t match[1];
    char buffer[256];
    char result[256];
    char *pos;

    regcomp(&regex, pattern, REG_EXTENDED);

    if (regexec(&regex, str, 1, match, 0) == 0) {
        // Copy the part before the match
        strncpy(buffer, str, match[0].rm_so);
        buffer[match[0].rm_so] = '\0';

        // Add the replacement
        strcpy(result, buffer);
        strcat(result, replacement);

        // Add the part after the match
        pos = str + match[0].rm_eo;
        strcat(result, pos);

        // Copy the result back to the original string
        strcpy(str, result);
    }

    regfree(&regex);
}

int main() {
    char str[100] = "The quick brown fox jumps over the lazy dog";
    const char *pattern = "[a-z]+";
    const char *replacement = "animal";

    replaceRegex(str, pattern, replacement);
    printf("Modified string: %s\n", str);

    return 0;
}

Output

Modified string: Tanimal quick brown fox jumps over the lazy dog

2 Replace Another Substring Matching a Regular Expression in a String

In this example,

We include the necessary headers regex.h for regular expression functions and string.h for string handling functions.
We create a function named replaceRegex to replace a substring matching a regular expression.
Inside the function, we compile the regular expression using regcomp.
We use regexec to find the substring that matches the pattern.
If a match is found, we copy the part before the match, add the new substring, and then add the part after the match to the result.
We return the modified string.
We call the replaceRegex function with the original string, the regex pattern to be replaced, and the new substring.
Finally, we print the modified string to the standard output.

C Program

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

void replaceRegex(char *str, const char *pattern, const char *replacement) {
    regex_t regex;
    regmatch_t match[1];
    char buffer[256];
    char result[256];
    char *pos;

    regcomp(&regex, pattern, REG_EXTENDED);

    if (regexec(&regex, str, 1, match, 0) == 0) {
        // Copy the part before the match
        strncpy(buffer, str, match[0].rm_so);
        buffer[match[0].rm_so] = '\0';

        // Add the replacement
        strcpy(result, buffer);
        strcat(result, replacement);

        // Add the part after the match
        pos = str + match[0].rm_eo;
        strcat(result, pos);

        // Copy the result back to the original string
        strcpy(str, result);
    }

    regfree(&regex);
}

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

    replaceRegex(str, pattern, replacement);
    printf("Modified string: %s\n", str);

    return 0;
}

Output

Modified string: The slow tortoise jumps over the lazy dog

Summary

In this tutorial, we learned How to Replace a Substring in a String using Regular Expression in C language with well detailed examples.

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