Got it, let's work through this date extraction problem together. You've got 7 specific valid date formats to target, so we can build a regex that covers each one with alternations, while avoiding partial matches or obvious invalid values like months >12 or days >31 (note: regex can't handle edge cases like February 30th—we'd need Python's datetime module for that level of strict validation).

The Regex Pattern #

Here's a regex that matches all your specified formats (line breaks are for readability; you can remove them for use in code):

\b(?:
    # MM/DD/YY or MM/DD/YYYY
    (?:0?[1-9]|1[0-2])/(?:0?[1-9]|[12]\d|3[01])/(?:\d{2}|\d{4})
    |
    # MM-DD-YYYY (with or without leading zero on month)
    (?:0?[1-9]|1[0-2])-(?:0?[1-9]|[12]\d|3[01])-\d{4}
    |
    # DD.MM.YYYY or DD. MM. YYYY (with optional spaces after dots)
    (?:0?[1-9]|[12]\d|3[01])\.(?:\s*0?[1-9]|1[0-2])\.\s*\d{4}
    |
    # YYYY-MM-DD
    \d{4}-(?:0?[1-9]|1[0-2])-(?:0?[1-9]|[12]\d|3[01])
)\b

Breakdown of Each Section #

Let's break down what each part does:

• \b: Word boundary to prevent partial matches (e.g., avoiding 123/45/678 being incorrectly picked up as a date)
• MM/DD/YY/YYYY:
  • (?:0?[1-9]|1[0-2]): Matches months 1-12, with or without a leading zero (e.g., 3 or 03)
  • (?:0?[1-9]|[12]\d|3[01]): Matches days 1-31, with or without a leading zero
  • (?:\d{2}|\d{4}): Matches 2-digit (e.g., 18) or 4-digit (e.g., 2018) years
• MM-DD-YYYY: Same month/day logic as above, but using - as the separator, with a fixed 4-digit year
• DD.MM.YYYY / DD. MM. YYYY: Swapped day/month order, using . as the separator (with optional spaces after dots to cover both formats)
• YYYY-MM-DD: 4-digit year first, followed by month and day with - separators

Python Implementation Example #

Here's how you can use this regex in your code to extract the valid dates:

import re

# Your test string
text_string = 'Examples for valid dates include "3/30/18", "3/30/2018", "3-30-2018", "03-30-2018", "30.3.2018", "30. 3. 2018", "2018-03-30". Some invalid ones: "13/30/2018", "3/32/2018", "2018/03/30"'

# Compile the regex (optional but efficient for repeated use)
date_regex = re.compile(r'\b(?:(?:0?[1-9]|1[0-2])/(?:0?[1-9]|[12]\d|3[01])/(?:\d{2}|\d{4})|(?:0?[1-9]|1[0-2])-(?:0?[1-9]|[12]\d|3[01])-\d{4}|(?:0?[1-9]|[12]\d|3[01])\.(?:\s*0?[1-9]|1[0-2])\.\s*\d{4}|\d{4}-(?:0?[1-9]|1[0-2])-(?:0?[1-9]|[12]\d|3[01]))\b')

# Extract all matches
valid_dates = date_regex.findall(text_string)

# Print results
print("Extracted valid dates:")
for date in valid_dates:
    print(f"- {date}")

Output #

Extracted valid dates:
- 3/30/18
- 3/30/2018
- 3-30-2018
- 03-30-2018
- 30.3.2018
- 30. 3. 2018
- 2018-03-30

Extra Tip for Strict Calendar Validation #

If you need to ensure the dates are actually calendar-valid (e.g., no February 30th), you can take the extracted strings and parse them with Python's datetime module:

from datetime import datetime

def is_valid_calendar_date(date_str):
    # Try all your formats to see if any parse successfully
    formats = ['%m/%d/%y', '%m/%d/%Y', '%m-%d-%Y', '%d.%m.%Y', '%d. %m. %Y', '%Y-%m-%d']
    for fmt in formats:
        try:
            datetime.strptime(date_str, fmt)
            return True
        except ValueError:
            continue
    return False

# Filter matches to only calendar-valid dates
strictly_valid_dates = [date for date in valid_dates if is_valid_calendar_date(date)]

内容的提问来源于stack exchange，提问作者Tiffany Tseng