$interest !~ /  # match against $interest (negated)
   ^     # start of line 
   -?    # optional minus sign
   \.?   # optional decimal point
   \d+   # 1 or more digits
   (?:     # start non-capturing group (i.e. not $1)
      \.     # decimal point
      \d+    # 1 or more digits
   )?      # end of group -- match 0 or 1 times
   $     # end of line
 /x; # end of regex (the /x modifier allows
     # comments in the regex -- can be useful)