Skip to content

Instantly share code, notes, and snippets.

@dhess

dhess/strrep.c

Created May 17, 2011 00:10
Show Gist options

  • Select an option

    Learn more about clone URLs
  • Save dhess/975639 to your computer and use it in GitHub Desktop.

Select an option

Learn more about clone URLs
Save dhess/975639 to your computer and use it in GitHub Desktop.
Download ZIP
strrep - substring replacement in C
Raw
strrep.c
/*
* strrep.c - C substring replacement.
*
* Written in 2011 by Drew Hess <dhess-src@bothan.net>.
*
* To the extent possible under law, the author(s) have dedicated all
* copyright and related and neighboring rights to this software to
* the public domain worldwide. This software is distributed without
* any warranty.
*
* For the full statement of the dedication, see the Creative Commons
* CC0 Public Domain Dedication at
* <http://creativecommons.org/publicdomain/zero/1.0/>.
*/
#include <stdlib.h>
#include <string.h>
#include <assert.h>
/*
* This file includes a main() function so that the file can be
* compiled into an executable, in order to run some simple test cases
* on the included strrep() function. If you want to use strrep in
* your own project, make sure you cut or comment out the main()
* function below.
*/
/*
* This function returns string s1 if string s2 is an empty string, or
* if s2 is not found in s1. If s2 is found in s1, the function
* returns a new null-terminated string whose contents are identical
* to s1, except that all occurrences of s2 in the original string s1
* are, in the new string, replaced by the string s3. The caller owns
* the new string.
*
* Strings s1, s2, and s3 must all be null-terminated strings. If any
* of s1, s2, or s3 are NULL, the function returns NULL, indicating an
* error condition. If any other error occurs, the function returns
* NULL.
*
* This code is written pedantically, primarily so that asserts can be
* used liberally. The code could certainly be optimized and/or made
* less verbose, and I encourage you to do that if you use strstr in
* your production code, once you're comfortable that it functions as
* intended. Each assert makes plain an invariant condition that is
* assumed to be true by the statement(s) that immediately follow the
* assert. Some of the asserts are trivially true, as written, but
* they're included, nonetheless, in case you, in the process of
* optimizing or adapting the code for your own purposes, make a
* change that breaks an assumption made downstream by the original
* code.
*/
char *
strrep(const char *s1, const char *s2, const char *s3)
{
if (!s1 || !s2 || !s3)
return 0;
size_t s1_len = strlen(s1);
if (!s1_len)
return (char *)s1;
size_t s2_len = strlen(s2);
if (!s2_len)
return (char *)s1;
/*
* Two-pass approach: figure out how much space to allocate for
* the new string, pre-allocate it, then perform replacement(s).
*/
size_t count = 0;
const char *p = s1;
assert(s2_len); /* otherwise, strstr(s1,s2) will return s1. */
do {
p = strstr(p, s2);
if (p) {
p += s2_len;
++count;
}
} while (p);
if (!count)
return (char *)s1;
/*
* The following size arithmetic is extremely cautious, to guard
* against size_t overflows.
*/
assert(s1_len >= count * s2_len);
assert(count);
size_t s1_without_s2_len = s1_len - count * s2_len;
size_t s3_len = strlen(s3);
size_t newstr_len = s1_without_s2_len + count * s3_len;
if (s3_len &&
((newstr_len <= s1_without_s2_len) || (newstr_len + 1 == 0)))
/* Overflow. */
return 0;
char *newstr = (char *)malloc(newstr_len + 1); /* w/ terminator */
if (!newstr)
/* ENOMEM, but no good way to signal it. */
return 0;
char *dst = newstr;
const char *start_substr = s1;
size_t i;
for (i = 0; i != count; ++i) {
const char *end_substr = strstr(start_substr, s2);
assert(end_substr);
size_t substr_len = end_substr - start_substr;
memcpy(dst, start_substr, substr_len);
dst += substr_len;
memcpy(dst, s3, s3_len);
dst += s3_len;
start_substr = end_substr + s2_len;
}
/* copy remainder of s1, including trailing '\0' */
size_t remains = s1_len - (start_substr - s1) + 1;
assert(dst + remains == newstr + newstr_len + 1);
memcpy(dst, start_substr, remains);
assert(strlen(newstr) == newstr_len);
return newstr;
}
/*
* A set of simple self-tests. To use strrep() in your own project,
* cut or comment out the lines that follow.
*
* Otherwise, to compile this file into a test program, do this:
*
* cc -o strrep strrep.c
*/
#include <stdio.h>
int
main(int argc, char **argv)
{
fprintf(stderr, "Test 1... ");
const char *s1 = 0;
const char *s2 = 0;
const char *s3 = 0;
assert(strrep(s1, s2, s3) == 0);
fprintf(stderr, "pass.\n");
fprintf(stderr, "Test 2... ");
s1 = "";
s2 = 0;
s3 = 0;
assert(strrep(s1, s2, s3) == 0);
fprintf(stderr, "pass.\n");
fprintf(stderr, "Test 3... ");
s1 = 0;
s2 = "";
s3 = 0;
assert(strrep(s1, s2, s3) == 0);
fprintf(stderr, "pass.\n");
fprintf(stderr, "Test 4... ");
s1 = "";
s2 = "";
s3 = 0;
assert(strrep(s1, s2, s3) == 0);
fprintf(stderr, "pass.\n");
fprintf(stderr, "Test 5... ");
s1 = 0;
s2 = 0;
s3 = "";
assert(strrep(s1, s2, s3) == 0);
fprintf(stderr, "pass.\n");
fprintf(stderr, "Test 6... ");
s1 = "";
s2 = 0;
s3 = "";
assert(strrep(s1, s2, s3) == 0);
fprintf(stderr, "pass.\n");
fprintf(stderr, "Test 7... ");
s1 = 0;
s2 = "";
s3 = "";
assert(strrep(s1, s2, s3) == 0);
fprintf(stderr, "pass.\n");
fprintf(stderr, "Test 8... ");
s1 = "";
s2 = "";
s3 = "";
assert(strrep(s1, s2, s3) == s1);
fprintf(stderr, "pass.\n");
fprintf(stderr, "Test 9... ");
s1 = "abc";
s2 = "";
s3 = "xyz";
assert(strrep(s1, s2, s3) == s1);
fprintf(stderr, "pass.\n");
fprintf(stderr, "Test 10... ");
s1 = "";
s2 = "abc";
s3 = "xyz";
assert(strrep(s1, s2, s3) == s1);
fprintf(stderr, "pass.\n");
fprintf(stderr, "Test 11... ");
s1 = "abc";
s2 = "def";
s3 = "xyz";
assert(strrep(s1, s2, s3) == s1);
fprintf(stderr, "pass.\n");
fprintf(stderr, "Test 12... ");
s1 = "ab";
s2 = "abc";
s3 = "xyz";
assert(strrep(s1, s2, s3) == s1);
fprintf(stderr, "pass.\n");
fprintf(stderr, "Test 13... ");
s1 = "abc";
s2 = "abc";
s3 = "xyz";
char *s4 = strrep(s1, s2, s3);
assert(strcmp(s4, "xyz") == 0);
assert(s4 != s1);
free(s4);
fprintf(stderr, "pass.\n");
fprintf(stderr, "Test 14... ");
s1 = "abc";
s2 = "a";
s3 = "xyz";
s4 = strrep(s1, s2, s3);
assert(strcmp(s4, "xyzbc") == 0);
free(s4);
fprintf(stderr, "pass.\n");
fprintf(stderr, "Test 15... ");
s1 = "abc";
s2 = "b";
s3 = "xyz";
s4 = strrep(s1, s2, s3);
assert(strcmp(s4, "axyzc") == 0);
free(s4);
fprintf(stderr, "pass.\n");
fprintf(stderr, "Test 15... ");
s1 = "abc";
s2 = "c";
s3 = "xyz";
s4 = strrep(s1, s2, s3);
assert(strcmp(s4, "abxyz") == 0);
free(s4);
fprintf(stderr, "pass.\n");
fprintf(stderr, "Test 16... ");
s1 = "aba";
s2 = "ab";
s3 = "xyz";
s4 = strrep(s1, s2, s3);
assert(strcmp(s4, "xyza") == 0);
free(s4);
fprintf(stderr, "pass.\n");
fprintf(stderr, "Test 17... ");
s1 = "bbc";
s2 = "bc";
s3 = "xyz";
s4 = strrep(s1, s2, s3);
assert(strcmp(s4, "bxyz") == 0);
free(s4);
fprintf(stderr, "pass.\n");
fprintf(stderr, "Test 18... ");
s1 = "a";
s2 = "a";
s3 = "xyz";
s4 = strrep(s1, s2, s3);
assert(strcmp(s4, "xyz") == 0);
free(s4);
fprintf(stderr, "pass.\n");
fprintf(stderr, "Test 19... ");
s1 = "ab";
s2 = "a";
s3 = "xyz";
s4 = strrep(s1, s2, s3);
assert(strcmp(s4, "xyzb") == 0);
free(s4);
fprintf(stderr, "pass.\n");
fprintf(stderr, "Test 20... ");
s1 = "ab";
s2 = "b";
s3 = "xyz";
s4 = strrep(s1, s2, s3);
assert(strcmp(s4, "axyz") == 0);
free(s4);
fprintf(stderr, "pass.\n");
fprintf(stderr, "Test 21... ");
s1 = "abbc";
s2 = "ab";
s3 = "x";
s4 = strrep(s1, s2, s3);
assert(strcmp(s4, "xbc") == 0);
free(s4);
fprintf(stderr, "pass.\n");
fprintf(stderr, "Test 22... ");
s1 = "abcc";
s2 = "bc";
s3 = "x";
s4 = strrep(s1, s2, s3);
assert(strcmp(s4, "axc") == 0);
free(s4);
fprintf(stderr, "pass.\n");
fprintf(stderr, "Test 23... ");
s1 = "dccd";
s2 = "cd";
s3 = "x";
s4 = strrep(s1, s2, s3);
assert(strcmp(s4, "dcx") == 0);
free(s4);
fprintf(stderr, "pass.\n");
fprintf(stderr, "Test 24... ");
s1 = "abab";
s2 = "ab";
s3 = "xyz";
s4 = strrep(s1, s2, s3);
assert(strcmp(s4, "xyzxyz") == 0);
free(s4);
fprintf(stderr, "pass.\n");
fprintf(stderr, "Test 25... ");
s1 = "abcab";
s2 = "ab";
s3 = "xyz";
s4 = strrep(s1, s2, s3);
assert(strcmp(s4, "xyzcxyz") == 0);
free(s4);
fprintf(stderr, "pass.\n");
fprintf(stderr, "Test 26... ");
s1 = "abcabc";
s2 = "ab";
s3 = "xyz";
s4 = strrep(s1, s2, s3);
assert(strcmp(s4, "xyzcxyzc") == 0);
free(s4);
fprintf(stderr, "pass.\n");
fprintf(stderr, "Test 27... ");
s1 = "cabcab";
s2 = "ab";
s3 = "xyz";
s4 = strrep(s1, s2, s3);
assert(strcmp(s4, "cxyzcxyz") == 0);
free(s4);
fprintf(stderr, "pass.\n");
fprintf(stderr, "Test 28... ");
s1 = "cabcabc";
s2 = "ab";
s3 = "xyz";
s4 = strrep(s1, s2, s3);
assert(strcmp(s4, "cxyzcxyzc") == 0);
free(s4);
fprintf(stderr, "pass.\n");
fprintf(stderr, "Test 29... ");
s1 = "abc";
s2 = "ab";
s3 = "ab";
s4 = strrep(s1, s2, s3);
assert(strcmp(s4, "abc") == 0);
free(s4);
fprintf(stderr, "pass.\n");
fprintf(stderr, "Test 30... ");
s1 = "abc";
s2 = "bc";
s3 = "bc";
s4 = strrep(s1, s2, s3);
assert(strcmp(s4, "abc") == 0);
free(s4);
fprintf(stderr, "pass.\n");
fprintf(stderr, "Test 31... ");
s1 = "abcc";
s2 = "abc";
s3 = "ab";
s4 = strrep(s1, s2, s3);
assert(strcmp(s4, "abc") == 0);
free(s4);
fprintf(stderr, "pass.\n");
fprintf(stderr, "Test 32... ");
s1 = "abccc";
s2 = "bc";
s3 = "b";
s4 = strrep(s1, s2, s3);
assert(strcmp(s4, "abcc") == 0);
free(s4);
fprintf(stderr, "pass.\n");
fprintf(stderr, "Test 33... ");
s1 = "abccc";
s2 = "cc";
s3 = "c";
s4 = strrep(s1, s2, s3);
assert(strcmp(s4, "abcc") == 0);
free(s4);
fprintf(stderr, "pass.\n");
fprintf(stderr, "Test 34... ");
s1 = "abcd";
s2 = "a";
s3 = "";
s4 = strrep(s1, s2, s3);
assert(strcmp(s4, "bcd") == 0);
free(s4);
fprintf(stderr, "pass.\n");
fprintf(stderr, "Test 35... ");
s1 = "abcd";
s2 = "bc";
s3 = "";
s4 = strrep(s1, s2, s3);
assert(strcmp(s4, "ad") == 0);
free(s4);
fprintf(stderr, "pass.\n");
fprintf(stderr, "Test 36... ");
s1 = "abcd";
s2 = "d";
s3 = "";
s4 = strrep(s1, s2, s3);
assert(strcmp(s4, "abc") == 0);
free(s4);
fprintf(stderr, "pass.\n");
fprintf(stderr, "Test 37... ");
s1 = "";
s2 = "";
s3 = "abc";
assert(strrep(s1, s2, s3) == s1);
fprintf(stderr, "pass.\n");
fprintf(stderr, "All tests pass.\n");
return 0;
}
@wissem01chiha
Copy link

wissem01chiha commented Aug 22, 2025

Thanks a lot for the function

Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment