Regular Expression Match to test for a valid year
The "accepted" answer to this question is both incorrect and myopic.
It is incorrect in that it will match strings like 0001
, which is not a valid year.
It is myopic in that it will not match any values above 9999. Have we already forgotten the lessons of Y2K? Instead, use the regular expression:
^[1-9]\d{3,}$
If you need to match years in the past, in addition to years in the future, you could use this regular expression to match any positive integer:
^[1-9]\d*$
Even if you don't expect dates from the past, you may want to use this regular expression anyway, just in case someone invents a time machine and wants to take your software back with them.
Note: This regular expression will match all years, including those before the year 1, since they are typically represented with a BC designation instead of a negative integer. Of course, this convention could change over the next few millennia, so your best option is to match any integer—positive or negative—with the following regular expression:
^-?[1-9]\d*$
Years from 1000 to 2999
^[12][0-9]{3}$
For 1900-2099
^(19|20)\d{2}$
You need to add a start anchor ^
as:
^\d{4}$
Your regex \d{4}$
will match strings that end with 4 digits. So input like -1234
will be accepted.
By adding the start anchor you match only those strings that begin and end with 4 digits, which effectively means they must contain only 4 digits.