Check if string matches pattern

python

string-validation

regex-patterns

pythonic-strings

byNikita Barsukov·Nov 17, 2024

Using the re module, re.fullmatch() ensures your string matches the regex pattern throughout. Here is an example of a U.S. Social Security Number (SSN) pattern:

import re

if re.fullmatch(r"\d{3}-\d{2}-\d{4}", "123-45-6789"):
    print("Aah! We found a valid SSN here.")  # Time to celebrate?, Nah...
else:
    print("Nope! This isn't an SSN.")

This quick check verifies if your string is a correctly formatted SSN. It is a Date of Birth in disguise or a just a number sequence, you tell me. \d stands for a digit, {n} is the multi-plix of it.

Pythonic string validation

When validating strings to match your custom pattern such as an uppercase letter followed by a sequence of digits, roll the dice this way:

import re  # Any Python party begins with an import

pattern = re.compile(r"^([A-Z][0-9]+)+$")
matches = pattern.fullmatch("A1B2")  # Let's check if the string is party-ready
if matches:
    print("Seems like someone studied regex. String is a match.")
else:
    print("Oops! The string doesn't adhere to your golden rules.")

In this model, "A1B2" Pierces Brosnan, "B10L1" Le Chiffre, and 'C1N200J1' Vesper Lynd, are all attending the party dressed up right. "a1B2", "A10B", or "AB400" apparently missed the dress code memo.

Unmasking the regex legend

In the regex jungle, ^([A-Z][0-9]+)+$ translates to uppercase letters' party followed by the digits' ball, all night long! [A-Z] invites any uppercase letter to the party, [0-9]+ allows one or more digits.+ keeps the madness going. ^ and $, the gatekeepers, make sure the party starts and ends right here.

Multiple roads to the regex paradise

While re.match() is a reliable partner, re.fullmatch() is your best friend when you want to check the full string.

Avoid tattooing incomplete patterns like "(^[A-Z]\d[A-Z]\d)", which strictly expects a letter-digit pair, twice. It's like expecting everyone to come in pairs to the party. Unfair!

Remember, re.search() can spy a pattern anywhere in the string, not necessarily from start to end. It's everywhere!