With a literal /č/g you'd have to worry about the file encoding (the file containing that code). If you look up the individual characters you want to replace in Unicode they'll be called "U+ABCD SOME TEXT DESCRIPTION" (where ABCD is a hex number). You can copy that number into a regex like
Given that č is U+010D, ć is U+0107, š is U+0161, đ is U+0111, and ž is U+0173,
will match those five characters. As for exactly what you want to do with this information, I'm not sure either.