Will it work for the convention in the UK of writing +44 (0)1234 567 8901 which says to use 01234 dialling code inside the UK or 441234 if calling from another country, and don’t dial 4401234 ever?
A truly global phone number regex is quite literally impossible to make. There are too many combinations and expectations built into these conventions. You listed several here.
The "best" solution is to separate country code into a different field or input. Then have everything other than the country code (generally called a "subscriber number") added to another input.
Then on the backend you would essentially strip out all the non-numeric characters from the subscriber number and combine the country code and stripped "subscriber number" into an E.164 format number and store that in the database.
(Source) I have spent a decade dealing with phone numbers in databases and web forms. This is the "best" way to handle it, and even it isn't bulletproof, but it works 99.8% of the time. The best way to handle the other 0.2% of cases is to make a descriptive error message that explains to the user how you are expecting them to input their number (ie. No extensions, etc).
