Considering these facts about phone number format:-

1. Country Code prefix starts with ‘+’ and has 1 to 3 digits
2. Last part of the number, also known as subscriber number is 4 digits in all of the numbers
3. Most of the countries have 10 digits phone number after excluding country code. A general observation is that all countries phone number falls somewhere between 8 to 11 digits after excluding country code.

String allCountryRegex = "^(\\+\\d{1,3}( )?)?((\\(\\d{1,3}\\))|\\d{1,3})[- .]?\\d{3,4}[- .]?\\d{4}$";

Let's break the regex and understand,

• ^ start of expression
• (\\+\\d{1,3}( )?)? is optional match of country code between 1 to 3 digits prefixed with '+' symbol, followed by space or no space.
• ((\\(\\d{1,3}\\))|\\d{1,3} is mandatory group of 1 to 3 digits with or without parenthesis followed by hyphen, space or no space.
• \\d{3,4}[- .]? is mandatory group of 3 or 4 digits followed by hyphen, space or no space
• \\d{4} is mandatory group of last 4 digits
• $ end of expression

This regex pattern matches most of the countries phone number format including these:-

        String Afghanistan      = "+93 30 539-0605";
        String Australia        = "+61 2 1255-3456";
        String China            = "+86 (20) 1255-3456";
        String Germany          = "+49 351 125-3456";
        String India            = "+91 9876543210";
        String Indonesia        = "+62 21 6539-0605";
        String Iran             = "+98 (515) 539-0605";
        String Italy            = "+39 06 5398-0605";
        String NewZealand       = "+64 3 539-0605";
        String Philippines      = "+63 35 539-0605";
        String Singapore        = "+65 6396 0605";
        String Thailand         = "+66 2 123 4567";
        String UK               = "+44 141 222-3344";
        String USA              = "+1 (212) 555-3456";
        String Vietnam          = "+84 35 539-0605";

Source:https://codingnconcepts.com/java/java-regex-for-phone-number/

Guess this regex would work:

^((0091)|(\+91)|(0))([7-9]{1})([0-9]{9})$

There are many variations possible for this problem. Here is a regular expression similar to an answer I previously placed on SO.

^\s*(?:\+?(\d{1,3}))?[-. (]*(\d{3})[-. )]*(\d{3})[-. ]*(\d{4})(?: *x(\d+))?\s*$

It would match the following examples and much more:

18005551234
1 800 555 1234
+1 800 555-1234
+86 800 555 1234
1-800-555-1234
1 (800) 555-1234
(800)555-1234
(800) 555-1234
(800)5551234
800-555-1234
800.555.1234
800 555 1234x5678
8005551234 x5678
1    800    555-1234
1----800----555-1234

Regardless of the way the phone number is entered, the capture groups can be used to breakdown the phone number so you can process it in your code.

• Group1: Country Code (ex: 1 or 86)
• Group2: Area Code (ex: 800)
• Group3: Exchange (ex: 555)
• Group4: Subscriber Number (ex: 1234)
• Group5: Extension (ex: 5678)

Here is a breakdown of the expression if you're interested:

^\s*                #Line start, match any whitespaces at the beginning if any.
(?:\+?(\d{1,3}))?   #GROUP 1: The country code. Optional.
[-. (]*             #Allow certain non numeric characters that may appear between the Country Code and the Area Code.
(\d{3})             #GROUP 2: The Area Code. Required.
[-. )]*             #Allow certain non numeric characters that may appear between the Area Code and the Exchange number.
(\d{3})             #GROUP 3: The Exchange number. Required.
[-. ]*              #Allow certain non numeric characters that may appear between the Exchange number and the Subscriber number.
(\d{4})             #Group 4: The Subscriber Number. Required.
(?: *x(\d+))?       #Group 5: The Extension number. Optional.
\s*$                #Match any ending whitespaces if any and the end of string.

To make the Area Code optional, just add a question mark after the (\d{3}) for the area code.

As long as the extension is always separated from the rest of the phone number, your regex will work fine. If there is no such separation, there is no way to correctly validate a phone number.

Also keep in mind that both extensions and phone numbers can vary in length from country to country, so there is no regex that will solve all cases. If you can produce a list of allowed extensions, you can work that into the regex and get better matches, but for many groups of arbitrary length of digits you will get many wrong matches.

I have simplified your regex a bit, so oyu can see @Dukeling's suggestions in practice. Your regex on top, mine on the bottom.

^(([+]|[0]{2})([\\d]{1,3})([\\s-]{0,1}))?([\\d]{10})$
^(  (\\+|00)    \\d{1,3}    [\\s-]?)?      \\d{10}  $