How to extract only the 7 digits(20-26) from a 27 digit number using parse rule set?

Summary: So we have a business case where we are getting 27 digit number in Additional Information field in a bank statement where we need to extract 7 digits starting (20-26) and disregard the last digit which is basically a check digit. The tricky part is that the 7 digit number which we are trying to extract (20-26) might have leading zeroes as well. How should the parse rule be configured in this case?