In IP address is any number from 0-255 followed by a period followed by any number from 0-255 followed by a period followed by any number from 0-255 followed by a period followed by any number from 0-255. If asked to define a regex to match an IP address, the student will often say [0-255].[0-255].[0-255].[0-255] which is incorrect for two reasons.
First, the regex to match "any number from 0-255" is not [0-255]. Literally, [0-255] means "match a 0 or a 1 or a 2 or a 5 or a 5". It will match a single character and only characters that are 0, 1, 2 or 5. Second, the period means "match any character". Here we see some examples of what this regex will and will not match.