Regular expressions to check for + sign
Solution 1:
Your first alternative allows only optional digits without any plus sign.
You can write the pattern as
^[0-9]*[+]$|^[+][0-9]*$|^[0-9]*[+][0-9]*$
Or shorten it to
^\d*\+\d*$
Note that both pattern allow only a + without any digits.
See a regex demo.