str replace and regex replace functions in C language

re_replace.h

#ifndef __RE_REPLACE__
#define __RE_REPLACE__

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

#ifdef __cplusplus
extern "C" {
#endif

char* re_replace(const char *s, regex_t *buffer, const char *rep, char **pout,
  	size_t *pout_sz);

#ifdef __cplusplus
}
#endif

char* re_replace(const char *s, regex_t *buffer, const char *rep, char **pout,
	  size_t *pout_sz) {
	int start = 0;
	size_t s_len = strlen(s);
	int range = s_len;
	size_t rep_len = strlen(rep);
	size_t out_sz = pout_sz ? (*pout_sz ? *pout_sz : 1) : 1;
	char* out = NULL;
	if (pout) {
		if (*pout) {
			*pout = (char*) realloc(*pout, out_sz * sizeof(char));
			memset(*pout, 0, out_sz);
		} else {
			*pout = (char*) calloc(out_sz, sizeof(char));
		}
		out = *pout;
	} else {
		out = (char*) calloc(out_sz, sizeof(char));
	}
	struct re_registers regs;
	while (true) {
		memset(&regs, 0, sizeof(re_registers));
		int res = re_search(buffer, s, s_len, start, range, &regs);
		if (res == -1) {
			if (out_sz - strlen(out) <= (size_t) range) {
				out_sz += range;
				out = (char*) realloc(out, out_sz);
				memset(out + strlen(out), 0, range);
			}
			strncat(out + strlen(out), s + start, range);
			break;
		} else if (res == -2) {
			fprintf(stderr, "re_search() returns -2 for an internal error.\n");
			break;
		} else {
			unsigned i;
			for (i = 0; i < regs.num_regs; i++) {
				if (regs.start[i] == -1)
					continue;
				size_t append_sz = regs.start[i] - start + rep_len;
				if (out_sz - strlen(out) <= append_sz) {
					out_sz += append_sz;
					out = (char*) realloc(out, out_sz);
					memset(out + strlen(out), 0, append_sz);
				}
				strncat(out + strlen(out), s + start, regs.start[i] - start);
				strncat(out + strlen(out), rep, rep_len);
				start = regs.end[i];
				range = s_len - regs.end[i];
			}
		}
	}
	if (pout_sz){
		*pout_sz = out_sz;
	}
	if (pout){
		*pout = out;
	}
	return out;
}

#endif

str_replace.h

#ifndef __STR_REPLACE__
#define __STR_REPLACE__

#include <string.h>

#ifdef __cplusplus
extern "C" {
#endif

int str_replace(const char *orig, const char *rep, const char *with,
    char **pbuf, size_t *pbuf_sz);

#ifdef __cplusplus
}
#endif

int str_replace(const char *orig, const char *rep, const char *with,
  	char **pbuf, size_t *pbuf_sz) {
	char *ins;    // the next insert point
	char *tmp;    // varies
	int len_rep;  // length of rep
	int len_with; // length of with
	int len_front; // distance between rep and end of last rep
	int count;    // number of replacements

	if (!orig)
		return -2;
	if (!rep || !(len_rep = strlen(rep)))
		return -2;
	if (!(ins = strstr(orig, rep)))
		return -1;
	if (!with)
		return -2;
	if (!pbuf)
		return -2;
	len_with = strlen(with);

	for (count = 0; ((tmp = strstr(ins, rep)) != NULL); ++count) {
		ins = tmp + len_rep;
	}
	size_t buf_sz = strlen(orig) + (len_with - len_rep) * count + 1;
	if (pbuf_sz)
		*pbuf_sz = buf_sz;
	if (*pbuf) {
		tmp = *pbuf = (char*) realloc(*pbuf, buf_sz);
		memset(*pbuf, 0, buf_sz);
	} else {
		tmp = *pbuf = (char*) calloc(buf_sz, sizeof(char));
	}

	while (count--) {
		ins = strstr(orig, rep);
		len_front = ins - orig;
		tmp = strncpy(tmp, orig, len_front) + len_front;
		tmp = strcpy(tmp, with) + len_with;
		orig += len_front + len_rep; // move to next "end of rep"
	}
	strcpy(tmp, orig);
	return 0;
}

#endif

@tanbro Thanks.