str_replace for C

Php has a useful function called str_replace which can search and replace a certain string in another big string. However there is no such function in C. So I wrote up one for myself. Here is the code.

/*
 * Search and replace a string with another string , in a string
 * */
char *str_replace(char *search , char *replace , char *subject)
{
	char  *p = NULL , *old = NULL , *new_subject = NULL ;
	int c = 0 , search_size;
	
	search_size = strlen(search);
	
	//Count how many occurences
	for(p = strstr(subject , search) ; p != NULL ; p = strstr(p + search_size , search))
	{
		c++;
	}
	
	//Final size
	c = ( strlen(replace) - search_size )*c + strlen(subject);
	
	//New subject with new size
	new_subject = malloc( c );
	
	//Set it to blank
	strcpy(new_subject , "");
	
	//The start position
	old = subject;
	
	for(p = strstr(subject , search) ; p != NULL ; p = strstr(p + search_size , search))
	{
		//move ahead and copy some text from original subject , from a certain position
		strncpy(new_subject + strlen(new_subject) , old , p - old);
		
		//move ahead and copy the replacement text
		strcpy(new_subject + strlen(new_subject) , replace);
		
		//The new start position after this search match
		old = p + search_size;
	}
	
	//Copy the part after the last search match
	strcpy(new_subject + strlen(new_subject) , old);
	
	return new_subject;
}

Usage

/*
 * @brief
 * PHP's str_replace ported to C
 * @author Silver Moon ([email protected])
 * */
 
#include<stdio.h>
#include<string.h>
#include<stdlib.h>

char *str_replace(char *search , char *replace , char *subject);

int main()
{
	char *a = "allblldddlll";
	char *c = str_replace("ll" , "xll" , a);
	puts(c);
	return 0;
}

/*
 * Search and replace a string with another string , in a string
 * */
char *str_replace(char *search , char *replace , char *subject)
{
	char  *p = NULL , *old = NULL , *new_subject = NULL ;
	int c = 0 , search_size;
	
	search_size = strlen(search);
	
	//Count how many occurences
	for(p = strstr(subject , search) ; p != NULL ; p = strstr(p + search_size , search))
	{
		c++;
	}
	
	//Final size
	c = ( strlen(replace) - search_size )*c + strlen(subject);
	
	//New subject with new size
	new_subject = malloc( c );
	
	//Set it to blank
	strcpy(new_subject , "");
	
	//The start position
	old = subject;
	
	for(p = strstr(subject , search) ; p != NULL ; p = strstr(p + search_size , search))
	{
		//move ahead and copy some text from original subject , from a certain position
		strncpy(new_subject + strlen(new_subject) , old , p - old);
		
		//move ahead and copy the replacement text
		strcpy(new_subject + strlen(new_subject) , replace);
		
		//The new start position after this search match
		old = p + search_size;
	}
	
	//Copy the part after the last search match
	strcpy(new_subject + strlen(new_subject) , old);
	
	return new_subject;
}






Output

axllbxlldddxlll
Last Updated On : 22nd December 2011

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3 Comments + Add Comment

  • c = ( strlen(replace) – search_size )*c + strlen(subject);

    //New subject with new size
    new_subject = malloc( c );

    Hallo
    This source is listed in Google so please correct

    If replace is shorter then search you get a negativ size –> segfault
    Check for nothing to replace.
    Free the old subject.

    By the way, i think strcat is easier then the strncpy way.

    Jürgen

  • Thanks for the function.

    Buf we need insert this new_subject[p-old] = ”; in line 53 and new_subject=”; in line 63

    • why do we need to insert that. inserting new_subject=” in line 63 will erase the new string.

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