eg: replace all "12" in "125671289312345" with "ab" or "a" or "abcd"
my patches: (one to gstring.h, one to gstring.c in glib-2.20.0, code tested.)



--- gstring.h	2009-03-13 12:09:59.000000000 +0800
+++ gstring.h2	2009-06-15 15:40:48.000000000 +0800
@@ -119,6 +119,9 @@
 GString*     g_string_erase	        (GString	 *string,
 					 gssize		  pos,
 					 gssize		  len);
+GString*     g_string_replace 		(GString     	 *string,
+		  			 const gchar 	 *sub,
+		 			 const gchar 	 *repl);
 GString*     g_string_ascii_down        (GString	 *string);
 GString*     g_string_ascii_up          (GString	 *string);
 void         g_string_vprintf           (GString	 *string,



--- gstring.c	2009-06-15 15:41:25.000000000 +0800
+++ gstring.c2	2009-06-15 15:41:00.000000000 +0800
@@ -1219,6 +1219,69 @@
 }
 


 /**
+ * g_string_replace:
+ * @string: a #GString
+ * @sub: the substring target in #GString, to be replaced.
+ * @repl: the third string to replace sub.
+ *
+ * Replaces all instances of @sub from a #GString with @repl.
+ * if @sub is empty or no instance found in #GString, nothing will happen.
+ * if @repl is empty, it will erase all @sub (s) in #GString.
+ *
+ * Returns: @string
+ */
+GString*
+g_string_replace (GString     *string,
+		  const gchar *sub,
+		  const gchar *repl)
+{
+  gsize ls, lr;
+  gchar *q, *t;
+
+  g_return_val_if_fail (string != NULL, NULL);
+  g_return_val_if_fail (sub != NULL, NULL);
+  g_return_val_if_fail (repl != NULL, NULL);
+
+  q = string->str;
+  ls = strlen(sub);
+  lr = strlen(repl);
+
+  if (!ls)
+	  goto end;
+
+  if(ls > lr) {
+	 while(*q != '\0' && (t = strstr(q,sub)) != NULL) {
+		memcpy(t, repl, lr);
+		strcpy(t+lr, t+ls);
+		q = t + lr;
+		string->len -= ls-lr;
+	  } 
+  } else if(ls == lr) {
+	 while(*q != '\0' && (t = strstr(q,sub)) != NULL) {
+		memcpy(t, repl, lr);
+		q = t + lr;
+	  }
+  } else {
+	 while(*q != '\0' && (t = strstr(q,sub)) != NULL) {
+  		ptrdiff_t offt, offq;
+
+		offq = q - string->str;
+		offt = t - string->str;
+		g_string_maybe_expand(string, lr-ls);
+  		q = string->str + offq;
+		t = string->str + offt;
+		g_memmove(t+lr, t+ls, strlen(t+ls)+1);
+		memcpy(t, repl, lr);
+		q = t + lr;
+		string->len += lr-ls;
+  	  }
+  }
+	
+end:
+  return string;
+}
+
+/**
  * g_string_ascii_down:
  * @string: a GString
  *