Python Regular Expression: Validate a Complete Identifier

Start with a small, explicit task

Validate fictional equipment labels such as QX-407. Reject extra surrounding text, lowercase letters and an incorrect digit count. The pattern is intentionally a small business rule for synthetic labels; it is not a universal identifier, email or security-validation expression.

Run the complete example

This original example was checked with Python 3.12.14. Save it as python_regular_expression.py and run it with Python 3.12. It uses only the standard library. The assertions check the stated results.

import re
pattern = re.compile(r"[A-Z]{2}-[0-9]{3}")
labels = ["QX-407", "qx-407", "QX-40", "prefix QX-407 suffix"]
valid = [pattern.fullmatch(label) is not None for label in labels]
assert valid == [True, False, False, False]
assert pattern.search(labels[-1]) is not None
assert pattern.fullmatch("QX-407\n") is None
print(valid)
print("substring found:", pattern.search(labels[-1]).group())

Explain the result

The first character class limits the two-letter prefix to A through Z. The digit class limits the numeric part to 0 through 9, and the braces specify exact counts. A hyphen outside a class is a literal separator here. The raw-string prefix is a useful habit for regex patterns because backslashes then reach the regex parser without an extra layer of ordinary string escaping.

The long fourth label fails fullmatch but succeeds with search because it contains a valid-looking fragment. Those results answer different questions. The final newline check also fails whole-value validation. If surrounding whitespace should be allowed, make normalization an explicit step and decide which characters may be removed before validating.

Avoid these mistakes

Do not replace [0-9] with a broader shorthand without checking whether the accepted character set changes. Avoid adding a permissive wildcard merely to make rejected examples pass. When a literal split or prefix check fully answers the task, a regular expression may be unnecessary.