what is the regular expression validation for mobile no should start with +91 and start with 7,8,9 numbers only.

\+91[7-9][0-9]*

You can also make "+" optional:

\+?91[7-9][0-9]*

Combination with the feature shown by Abhinav will add constraints of start and end of line and number of digits (8 or any other):

^\+?91[7-9][0-9]{8}$

Or you can add optional blank space or "-". Do you need this as well? The idea is the same.

—SA