Here are five practice problems designed to help you get hands$-$on experience writing regular expressions in a Ruby program. In these problems, complete the given code by inserting the appropriate regular expression to fulfill the task described in each problem.

$1.$ Extracting Hashtags from a Tweet

Replace nil with the regular expression to extract all hashtags.

$2.$ Validating a Password

Replace nil with the regex for a password that must contain at least one uppercase letter, one lowercase letter, one digit, and be $8$ to $16$ characters long.

$3.$ Finding Numeric Values in a String

Replace nil with the regex to find all numeric values in a string.

$4.$ Matching a Date Format

Replace nil with the regex that matches dates in the format YYYY$-$MM$-$DD$.$

$5.$ Parsing a Log Entry

Replace nil with the regex to match log entries beginning with a severity level $($INFO$,$ WARN, or ERROR$),$ followed by a message.