C Pointer operation--replace a character or substring in a string

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

#include <stdlib.h>

Replaces all S1 characters in the source string with the character S2 void Replace_char (char *result, Char *source, char S1, char S2)
{
int i = 0;
char *q = NULL;

Q = source;
for (i=0; I<strlen (q); i++)
{
if (q[i] = = S1)
{
Q[i] = s2;
}
}
strcpy (result, q);
}


void Replace_string (char *result, Char *source, char* S1, Char *s2)
{
Char *q=null;
Char *p=null;

P=source;
while ((Q=strstr (P, S1))!=null)
{
strncpy (result, p, q-p);
result[q-p]= '//very '; important, must attention!
strcat (result, S2);
strcat (Result, Q+strlen (S1));
strcpy (P,result);
}
strcpy (result, p);
}

void Main ()
{
Char url[64] = "Ad12 fdf5 d12f dfd12 DFP";
Char result[64] = {0};

Replace_char (result, url, ', ' _ ');//Replace All ' characters in URL with ' _ ' characters, resulting in
printf ("Char final result=%s\n", result);
memset (result, 0, sizeof (result));
Replace_string (result, URL, "12", "@@@");//Replace All "12" strings in the URL with a "@@@" string
printf ("String final result=%s\n", result);

}

The results of running separately are as follows:

Test Result: Char final RESULT=AD12_FDF5_D12F_DFD12_DFP

Test results: string:final result=ad@@@ fdf5 d@@@f dfd@@@ DFP