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JavaScript

Posted by Bashar Ghadanfar
Bashar Ghadanfar
28,309 Points

When using lookaheads, why wouldn't + work instead of .*

In the "Validating a Password" lesson of the Regular Expression course, the expected password must contain a lowercase letter, an uppercase letter, and a number. https://teamtreehouse.com/library/validating-a-password .

Why would this regex work:

/^(?=.*\d)(?=.*[a-z])(?=.*[A-Z]).*$/

but this wouldn't:

/^(?=\d+)(?=[a-z]+)(?=[A-Z]+).*$/

Why?

1 Answer

Steven Parker
Steven Parker
243,266 Points
Steven Parker
Steven Parker
243,266 Points

You forgot to provide a link to the course page, so I'm not sure what the exercise criteria are. But these specify rather different requirements. Just looking at the meanings of one part, for example:

  • .*[a-z] :point_left: accept any number of any kind of characters followed by a lower-case letter
  • [a-z]+ :point_left: accept only one or more lower-case letters
Bashar Ghadanfar
Bashar Ghadanfar
28,309 Points

This is the lesson: https://teamtreehouse.com/library/validating-a-password .

Thanks for the response, Steven Parker . I still don't understand why my solution doesn't pass the password validation though.

Steven Parker
Steven Parker
243,266 Points
Steven Parker
Steven Parker
243,266 Points

As I said, the first pattern matches if the character (lower case letter in this example) is found after any number of other characters, so it can be at any position. But the second pattern matches only if the very first character (and any number of others) matches.

Since the first character cannot be lower case and upper case and a number, the second regex rejects everything.