Angular Pattern Validation for Sri Lankan phone numbers

I couldn't find correct pattern validation for Sri Lankan phone numbers; The formats are:

1. 711234567 (9 digits)
2. 0711234567 (10 digits)
3. +94711234567

It should start with 7 or 0 or +94. Can anyone help me for solving this. Appreciate your solutions. (I use reactive form for get data)