RegEx

The re module

A regular expression describes a shape of text. re matches those shapes:

Python

import re

text = "call 0176 1234567 or 0151 7654321"
print(re.findall(r"\d+", text))

Output

['0176', '1234567', '0151', '7654321']

Always write patterns as raw strings. Without the r, Python processes the backslashes before re ever sees them.

The pieces worth knowing

Pattern	Matches
`\d`	a digit
`\w`	a letter, digit or underscore
`\s`	whitespace
`.`	any character except newline
`[abc]`	one of a, b or c
`[^abc]`	anything except those
`+`	one or more
`*`	zero or more
`?`	zero or one
`{2,4}`	between two and four
`^` / `$`	start / end of the string
`\b`	a word boundary

Python

import re

print(re.findall(r"\b\w{5}\b", "the quick brown fox jumps"))
print(re.findall(r"[aeiou]", "programming"))
print(re.findall(r"^\w+", "first word only"))

Output

['quick', 'brown', 'jumps']
['o', 'a', 'i']
['first']

The main functions

Python

import re

text = "Ada was born in 1815"

print(re.search(r"\d{4}", text).group())
print(re.findall(r"\w+", text))
print(re.sub(r"\d{4}", "YYYY", text))
print(re.split(r"\s+", text))

Output

1815
['Ada', 'was', 'born', 'in', '1815']
Ada was born in YYYY
['Ada', 'was', 'born', 'in', '1815']

search finds the first match anywhere, or None
match only matches at the start
findall gives every match as a list
sub replaces
split breaks on the pattern

search returns None when there is nothing

Python

import re

found = re.search(r"\d+", "no digits here")
print(found)

if found:
    print(found.group())
else:
    print("nothing matched")

Output

None
nothing matched

Calling .group() on that None raises AttributeError, which is the most common regex mistake. Check first.

Capture groups

Brackets capture part of the match:

Python

import re

match = re.search(r"(\d{2})/(\d{2})/(\d{4})", "dated 14/03/2026 exactly")
print(match.group(0))
print(match.group(1))
print(match.groups())

Output

14/03/2026
14
('14', '03', '2026')

Group 0 is the whole match; the rest are the brackets, left to right.

Named groups read far better once there are more than two:

Python

import re

pattern = r"(?P<day>\d{2})/(?P<month>\d{2})/(?P<year>\d{4})"
match = re.search(pattern, "14/03/2026")
print(match.group("year"))
print(match.groupdict())

Output

2026
{'day': '14', 'month': '03', 'year': '2026'}

With groups, findall returns the groups rather than the whole match:

Python

import re
print(re.findall(r"(\w+)@(\w+)\.com", "a@x.com b@y.com"))

Output

[('a', 'x'), ('b', 'y')]

Greedy by default

+ and * take as much as they can. Add ? to take as little:

Python

import re

html = "<b>bold</b> and <i>italic</i>"
print(re.findall(r"<.+>", html))
print(re.findall(r"<.+?>", html))

Output

['<b>bold</b> and <i>italic</i>']
['<b>', '</b>', '<i>', '</i>']

The greedy version ran from the first < to the last >. This surprises everyone once.

Flags

Python

import re

print(re.findall(r"ada", "Ada and ADA", re.IGNORECASE))
print(re.sub(r"\s+", " ", "too    much\n\nspace"))

Output

['Ada', 'ADA']
too much space

Compile a pattern you reuse

Python

import re

word = re.compile(r"\b[A-Z]\w+")
print(word.findall("Ada met Grace in London"))
print(word.sub("X", "Ada met Grace"))

Output

['Ada', 'Grace', 'London']
X met X

When not to use it

A regex is powerful and hard to read. If a string method does the job, use it:

Python

name = "report.pdf"
print(name.endswith(".pdf"))
print(name.split("."))

Output

True
['report', 'pdf']

The re module

The pieces worth knowing

The main functions

search returns None when there is nothing

Capture groups

Greedy by default

Flags

Compile a pattern you reuse

When not to use it

Test yourself

Why should regex patterns be raw strings?

What does re.search return when nothing matches?

Why does <.+> match the whole string rather than one tag?

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