Question

What does the following regular expression do?

^\d{1,2}(\-|\/|\.)\d{1,2}\1\d{4}$

Particularly, what is the purpose of the \1 part?

Answer 1

You should check out a tutorial. There’s barely anything advanced in there (well except for that \1 you pointed to):

^         # the start of the string
\d        # a digit
{1,2}     # 1 or 2 of those
(         # start the first subpattern, which can later be referred to with \1
  \-      # a literal hyphen (there is no need for escaping, but it doesn't hurt)
|         # or
  \/      # a literal slash
|         # or
  \.      # a literal period
)         # end of subpattern
\d{1,2}   # one or two more digits
\1        # the exact same thing that was matched in the first subpattern. this
          # is called a backreference
\d{4}     # 4 digits
$         # the end of the string

I.e. this asserts that the input string contains exactly one date, no more no less, of format ddmmyyyy (or could also be mmddyyyy), with possible delimiters ., - or / (and consistent delimiter usage). Note that it does not ensure a correct date. Months and days could be anything from 00 to 99.

Note that the exact meaning of \d depends on the regex engine and culture you are using. Usually it means [0-9] (any ASCII digit). But for example in .NET it can also mean “any Unicode character that represents a digit”.