Python – Validate String date format

Given a date format and a string date, the task is to write a python program to check if the date is valid and matches the format.

Examples:

Input : test_str = ’04-01-1997′, format = “%d-%m-%Y”

Output : True

Explanation : Formats match with date.

Input : test_str = ’04-14-1997′, format = “%d-%m-%Y”

Output : False

Explanation : Month cannot be 14.

Method #1 : Using strptime()

In this, the function, strptime usually used for conversion of string date to datetime object, is used as when it doesn’t match the format or date, raises the ValueError, and hence can be used to compute for validity.

Output:

The original string is : 04-01-1997
Does date match format? : True

Method #2 : Using dateutil.parser.parse()

In this, we check for validated format using different inbuilt function, dateutil.parser. This doesn’t need the format to detect for a date.

Output:

The original string is : 04-01-1997
Does date match format? : True

Method#3: Using regular expression

Approach

validate a string date format is by using regular expressions. We can define a regular expression pattern that matches the expected format, and then use the re module to check if the string matches the pattern.

Algorithm

1. Import the re module
2. Define the input test string and the regular expression pattern string
3. Use the re.match() method to match the pattern against the test string
4. If the pattern matches the test string:
a. Print “True”
5. Otherwise:
a. Print “False”

True

Time complexity of this approach is O(n), where n is the length of the input string. 

Auxiliary Space is also O(n), since we need to store the regular expression pattern in memory.