Validate Indian Phone number with optional +91 or 0 preceeding 10 digits

This should do

^(0|\+91)?[789]\d{9}$

? matches preceding character or group optionally..

You can use ^([0]|\+91)?\d{10} as starts with 0 or +91 and 10 digits after that.

..but as you must have seen number starts with 7,8 or 9

Then, you should use ^([0]|\+91)?[789]\d{9}$ which means starts with 7, 8 or 9 and follows 9 digits after that.

Some random matches;

+919802422462
08150166859