I tried to make a regular expression of "date" using Python

Conclusion

Here is the regular expression for "date" in python.

The year version is as follows.

pattern = r'[12]\d{3}[/\-Year](0?[1-9]|1[0-2])[/\-Month](0?[1-9]|[12][0-9]|3[01])Day?$'
# OK
#February 22, 2020
# 2020-2-22
# 2020/2/22
# 1985/01/12
# 2010/12/11
# 2022/02/22

# NG
# 9999/99/99

The Japanese calendar version is as follows.

pattern = r'(Meiji|Taisho|Showa|Heisei|Reiwa)\d{1,2}Year(0?[1-9]|1[0-2])Month(0?[1-9]|[12][0-9]|3[01])Day'
# OK
#Reiwa February 22, 2002
#February 22, 2nd year of Reiwa
#February 22, 1990
#February 22, 1945
#February 22, 1918
#February 22, 1897

# NG
#Reiwa September 99, 1999

Preparation

The environment uses Google Colaboratory. The Python version is below.

import platform
print("python " + platform.python_version())
# python 3.6.9

The regular expression check tool used: https://regex101.com/ While checking here, we will create a regular expression and implement it in the code.

Also, this is easy to understand about Python regular expressions in general. https://qiita.com/luohao0404/items/7135b2b96f9b0b196bf3

Let's create a date regular expression

Year version

Let's write the code immediately. First, import the library for using regular expressions.

import re

First of all 2022/02/22 Let's create a regular expression that matches the string.

pattern = r'2022/02/22'

Of course, this is an exact match, so it matches. Let's check with the code.

pattern = r'2022/02/22'
string = r'2022/02/22'
prog = re.compile(pattern)
result = prog.match(string)
if result:
    print(result.group())
# 2022/02/22

The matched string is displayed. After that, for the sake of simplicity, only the regular expression pattern is described.

In addition to "2022/02/22", there are other dates such as "1985/01/12" and "2010/12/11". The regular expressions that match these are as follows.

pattern = r'\d\d\d\d/\d\d/\d\d'

The regular expression used is:

The regular expression above can be expressed more easily.

pattern = r'\d{4}/\d{2}/\d{2}'

The newly used regular expressions are:

However, this will result in an impossible date string, such as "9999/99/99". This time, we will allow only the following conditions as the YYYY / MM / DD format.

• YYYY：1000~2999
• MM：01~12
• DD：01~31

The modified regular expression is as follows.

pattern = r'[12]\d{3}/(0[1-9]|1[0-2])/(0[1-9]|[12][0-9]|3[01])'

The newly used regular expressions are:

We also used the following regular expressions.

You now have a regular expression that matches only the above conditions.

However, with this, things that are not 0-filled (0 padded), such as "2020/2/22", cannot be taken. The modified regular expression is as follows.

pattern = r'[12]\d{3}\/(0?[1-9]|1[0-2])\/(0?[1-9]|[12][0-9]|3[01])$'

The newly used regular expressions are:

We also used the following regular expressions.

Without this, "2022/02/22" will only match until "2022/02/2".

With this, it is possible to handle the one without 0 padding (0 padding).

Furthermore, let's modify it so that it matches not only "/ (slash)" but also "-(hyphen)" and "year / month (day)".

pattern = r'[12]\d{3}[/\-Year](0?[1-9]|1[0-2])[/\-Month](0?[1-9]|[12][0-9]|3[01])Day?$'

Here, "\-" is an escape, which means that "-(slash)" is not used in a special meaning but is a character.

Now you have a regular expression that matches not only "/ (slash)" but also "-(hyphen)" and "year / month (day)".

Japanese calendar version

Dates include not only the Western calendar but also Japanese calendar dates such as "February 22, 2nd year of Reiwa", so let's create a regular expression here as well.

Consider the following as conditions for dates in the Japanese calendar. --A character string that starts with any of Meiji, Taisho, Showa, Heisei, and Reiwa. --The year is a two-digit number. There is no such thing as "1999", but this time it is acceptable. --The numbers are separated only by "year / month / day". Excludes "/ (slash)" and "-(hyphen)".

The regular expression is:

pattern = r'(Meiji|Taisho|Showa|Heisei|Reiwa)\d{1,2}Year(0?[1-9]|1[0-2])Month(0?[1-9]|[12][0-9]|3[01])Day'

Summary

This time, I used Python to create a regular expression for "date".

Character strings with a certain pattern, such as dates, times, and amounts, are compatible with regular expressions. Try to extract various character strings with regular expressions.