import re
regex = re.compile(r"""
^
# Validate the basic structure
(?=\d{1,4}\/\d{1,4}\/\d{1,4}$)
# The below subexpression matches a leap year
# This will be useful later when checking Feb 29th
(?<leap>0*$|\d*(?:[13579][26]|(?:\b|[02468])[048])(?:00|(?<!00))$){0}
# Match and capture a month, saving a 30-day month or Feb for later reference
0*(?<month>(?<thirty>9|4|6|11)|(?<feb>2)|1[02]|[13578])\/
# If Feb was matched and 29 appears, check for a leap year
0*(?<day>[1-9]|1\d|2[0-8]|(?(feb)29(?=\/(?&leap))|(?:29|(?(thirty)30|3[01]))))\/
# Match and capture a year
(?:0\B)*(?<year>\d+)
$
""", flags=re.VERBOSE | re.MULTILINE)
test_str = ("1/25/2018\n"
"3/11/2119\n"
"6/8/224\n"
"6/54/1996\n"
"2/29/2004\n"
"2/29/1900")
matches = regex.finditer(test_str)
for match_num, match in enumerate(matches, start=1):
print(f"Match {match_num} was found at {match.start()}-{match.end()}: {match.group()}")
for group_num, group in enumerate(match.groups(), start=1):
print(f"Group {group_num} found at {match.start(group_num)}-{match.end(group_num)}: {group}")
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for Python, please visit: https://docs.python.org/3/library/re.html