Regex number validation isolating phone number and country code

Question

I am pretty much entirely new to writing regex, but i have gotten a task where i must create a phone number validation system.

Currently it is a bit of a headache for me.

My current progress has led me to:

^([+]45|[(][+]45[)]?|[(]0045[)]|0045)?\s*([0-9]{8})$

There are two things i need to achieve:

1. Country code. The country code can be written in various formats.

   1. With a plus preceding the country code
   2. With a plus preceding the country code but in parantheses
   3. With 00 preceding the country code
   4. With 00 preceding the country code but in parantheses

2. The Phone number The phone number is assumed to always be from my country (which is why the country code always
   the same as well

The issue comes when 00 is used for the country code. While I want to require the phone number to be with 8 digits, when the country code is 0045, only 4 digits for the phone number are required to get a "true" result.

I just cannot figure out how to ensure that this will be the case. Can this even be done in a reasonable way?

Answer 1

To match 8 digits for the phone number excluding the prefix, you can match either +45 or (45) or (0045) or 0045 followed by 8 digits.

Or match 8 digits asserting the number does not start with 00 for a country code using a negative lookahead (?!00)

^(?:\+45|\(\+45\)|\(0045\)|0045|(?!00))\d{8}$

Explanation

• ^ Start of string
• (?: Non capture group
  • \+45|\(\+45\)|\(0045\)|0045|(?!00) Match either of the alternations, or assert not 00 at the start of the string
• ) Close non capture group
• \d{8} Match 8 digits
• $ End of string

Regex demo

Answer 2

I have altered the expression to: ^(^([+]45|[(][+]45[)]?|[(]0045[)]|0045)?\s*([1-9]{1})([0-9]{7})$

Which works for my purpose

But better solutions might be required later as international numbers might also be accepted at a later point.