Matching anything else but a date - regex

Question

I'm using the REGEX below to effectively check if a string is a YYYY-MM-DD date.
[0-9]{4}-[0-9]{2}-[0-9]{2}
How do I do the reverse and use a similar REGEX to check a string is NOT a date in this format.

Answer 1

You could use a negative look-ahead to solve this:
(?![0-9]{4}-[0-9]{2}-[0-9]{2})
Edit: After watching the post by Lucasus I've made a new regex to have a more strict validation.
Year can be any combination of 4 digits, which e.g. allows dates pre 1900
The month is in the range of 1-12
Days in the range of 1-31,
Validates in the format YYYY-MM-DD
New Regex:
(?!([0-9]{4})-([1-9]|0[1-9]|1[012])-([1-9]|0[1-9]|[12][0-9]|3[01]))

Answer 2

Your regex matches more than only a valid date (for example "3333-99-99"), You can use a longer expression:
^(19[0-9][0-9]|20[0-9][0-9])-([1-9]|0[1-9]|1[012])-([1-9]|0[1-9]|[12][0-9]|3[01])$
If you want match everything but the date, use negative look-ahead, as Marcus wrote:
^(?!((19[0-9][0-9]|20[0-9][0-9])-([1-9]|0[1-9]|1[012])-([1-9]|0[1-9]|[12][0-9]|3[01])))$
The regex is from this link

Answer 3

import datetime
try:
datetime.datetime.strptime('2011-10-27','%Y-%m-%d')
except ValueError:
print 'The string is NOT in the right format'
else:
print 'The string is in the right format'
Indeed this isn't regex, but it might perform better - may be worth benchmarking...

Answer 4

First, your regex doesn't ensure that the string is a date, just that it contains one. If you wanted to make sure the string contains nothing but a date (according to your formulation), you would need to anchor it:
^[0-9]{4}-[0-9]{2}-[0-9]{2}$
...and the simplest way to ensure that the string that is not a date is to try to match it with that regex and negate the result. How you do that depends on the language; in C# you could do this:
if ( !Regex.IsMatch(s, "^[0-9]{4}-[0-9]{2}-[0-9]{2}$") )
If you must do the check with the regex itself (for example, if you're using a simple regex-based validation control), you can use a negative lookahead, as other responders advised:
^(?![0-9]{4}-[0-9]{2}-[0-9]{2}$).*
Starting from the beginning of the string (because of the ^), the lookahead tries to match a date followed by the end of the string ($). If that fails, the match position is reset to the beginning, and the .* goes ahead and consumes the whole string.