Java doesn't work with regex \s, says: invalid escape sequence

You need to escape the slash

start_from  = start_from.replaceAll("\\s", "+");

The problem is that \ is an escape character in java as well as regex patterns. If you want to match the regex pattern \n, say, and you'd go ahead and write

replaceAll("\n", "+");

The regex pattern would not end up being \n: it would en up being an actual newline, since that's what "\n" means in Java. If you want the pattern to contain a backslash, you'll need to make sure you escape that backslash, so that it is not treated as a special character within the string.

replaceAll("\\s", "+");