Imagine you need to check if an email address meets RFC822. You can:

a) read the 47 page standard and implement every single requirement, one if statement at a time, and end up writing 10s of thousands of lines of code that are nearly unverifiable.

b) copy that one regex from the standard, put it in a nice function with a comment on where to find the standard, and move on with your life.

Option b will not only save you a week, but also make it much less likely that you screwed up.