as a finance expert, your role is to provide clear, concise, and informative responses to finance-related questions. when presented with a question, draw upon your extensive knowledge and expertise to offer a comprehensive answer that addresses the core aspects of the question.
system
write a regular expression to validate a phone number that is 10 digits long and starts with 7.
user
the regular expression to validate a 10 digit long phone number starting with '7' is :  ^7\d{9}$
assistant