We want to extract the numbers from the strings stored in s. However, there a lot of non-numeric characters that will get in the way. We can remove these, but before doing this we want to preserve the...

We want to extract the numbers from the strings stored in s. However, there a lot of non-numeric characters that will get in the way. We can remove these, but before doing this we want to preserve the string with the column header, which includes the month abbreviation. Use the str_which function to find the rows with a header. Save these results to header_index. Hint: find the first string that matches the pattern 2015 using the str_which function.