Function add_value

3rd/lua-5.4.3/src/lstrlib.c:906–939

** Add the replacement value to the string buffer 'b'. ** Return true if the original string was changed. (Function calls and ** table indexing resulting in nil or false do not change the subject.) */

** table indexing resulting in nil or false do not change the subject.)
*/
static int add_value (MatchState *ms, luaL_Buffer *b, const char *s,
                                      const char *e, int tr) {
  lua_State *L = ms->L;
  switch (tr) {
    case LUA_TFUNCTION: {  /* call the function */
      int n;
      lua_pushvalue(L, 3);  /* push the function */
      n = push_captures(ms, s, e);  /* all captures as arguments */
      lua_call(L, n, 1);  /* call it */
      break;
    }
    case LUA_TTABLE: {  /* index the table */
      push_onecapture(ms, 0, s, e);  /* first capture is the index */
      lua_gettable(L, 3);
      break;
    }
    default: {  /* LUA_TNUMBER or LUA_TSTRING */
      add_s(ms, b, s, e);  /* add value to the buffer */
      return 1;  /* something changed */
    }
  }
  if (!lua_toboolean(L, -1)) {  /* nil or false? */
    lua_pop(L, 1);  /* remove value */
    luaL_addlstring(b, s, e - s);  /* keep original text */
    return 0;  /* no changes */
  }
  else if (l_unlikely(!lua_isstring(L, -1)))
    return luaL_error(L, "invalid replacement value (a %s)",
                         luaL_typename(L, -1));
  else {
    luaL_addvalue(b);  /* add result to accumulator */
    return 1;  /* something changed */
  }
}


static int str_gsub (lua_State *L) {