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.