matching regex flutter code example

Example: flutter regex validation

You could make the first part optional matching either a + or 0 followed by a 9. Then match 10 digits:

^(?:[+0]9)?[0-9]{10}$
^ Start of string
(?:[+0]9)? Optionally match a + or 0 followed by 9
[0-9]{10} Match 10 digits
$ End of string
Regex demo