Flutter - Validate a phone number using Regex

Validation using Regex:

String validateMobile(String value) {
    String pattern = r'(^(?:[+0]9)?[0-9]{10,12}$)';
    RegExp regExp = new RegExp(pattern);
    if (value.length == 0) {
          return 'Please enter mobile number';
    }
    else if (!regExp.hasMatch(value)) {
      return 'Please enter valid mobile number';
    }
    return null;
}                                                                                                  

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