Regular Expressions: Finding Patterns

Why this matters

String methods work when you know exactly what to look for. But what about “any phone number”, “every hashtag” or “a date in the form 2026-11-06”? A regular expression (regex) describes a pattern of text, and Python’s re module finds, extracts and replaces every match. It is a compact language worth learning at a basic level.

The idea

import re

text = "Order 66 shipped on 2026-11-06"
re.findall(r"\d+", text)             # ['66', '2026', '11', '06']
re.search(r"\d{4}", text).group()    # '2026'  (the first match, or None)
re.sub(r"\d", "#", text)             # replace every digit with #

Always write patterns as raw strings (r"...") so Python leaves the backslashes alone.

The building blocks:

Pattern	Matches
\d / \w / \s	a digit / a letter, digit or underscore / whitespace
.	any one character
+ / * / ?	one or more / zero or more / optional
{4}	exactly four of the previous thing
[abc] / [a-z]	any one of those characters
^ / $	start / end of the text
( )	a group: capture just that part

With groups, findall returns only the captured part, and match.groups() gives them all:

m = re.fullmatch(r"(\d{4})-(\d{2})", "2026-11")
m.groups()                             # ('2026', '11')

re.fullmatch requires the whole text to match; re.search finds a match anywhere. They return None when there is no match, so check before using the result.

Try it

Use re.findall to pull every word that starts with a capital letter out of a sentence ([A-Z]\w*). Then use re.sub to replace runs of spaces with a single space.

Your turn

Write three functions:

• find_numbers(text): every whole number in the text, as a list of ints: find_numbers("3 cats and 12 dogs") is [3, 12].
• find_hashtags(text): the words after each #, without the #: find_hashtags("I love #python and #coding!") is ["python", "coding"].
• parse_date(text): if text is exactly YYYY-MM-DD return (year, month, day) as ints, otherwise return None. parse_date("2026-11-06") is (2026, 11, 6).

Recap

• re.findall, re.search, re.fullmatch and re.sub find, test and replace patterns.
• Write patterns as raw strings; \d, +, {n}, [ ] and ( ) cover most needs.
• A failed search or match gives None: check before calling .group().

Common mistakes

• Forgetting the r prefix, so "\d" is treated as a broken escape.
• Using .group() on None: AttributeError: 'NoneType' object has no attribute 'group'.
• Regex for everything. If split or startswith does the job, use that: it is easier to read.