Programming Fundamentals › Programming Basics
Regular Expression
A pattern language for matching and extracting text.
Also known as: regex, regexp
A regular expression (regex) is a compact pattern for matching and extracting text.
import re
re.search(r"\d{4}-\d{2}-\d{2}", "Due 2026-10-10") # finds a date
re.findall(r"[\w.]+@[\w.]+", "a@x.com, b@y.org") # ['a@x.com', 'b@y.org']
re.sub(r"\s+", " ", "too many spaces") # 'too many spaces'The building blocks
| Pattern | Matches |
|---|---|
abc | the literal text |
. | any one character |
\d \w \s | a digit, a word character, whitespace |
[aeiou] | one character from the set |
* + ? | zero or more, one or more, optional |
{3} {2,4} | an exact or ranged count |
^ $ | start and end of the text |
( ) | a group, whose match you can capture |
a|b | a or b |
m = re.match(r"(\d+)-(\w+)", "42-abc")
m.group(1) # "42"Habits
- Test patterns interactively with a regex tool before putting them in code (regex tools).
- Anchor them (
^...$) when validating a whole value, or"abc"will match inside"xabcx". - Don’t parse HTML or complex formats with regex. Use a real parser.
- Be wary of emails and URLs. Perfect patterns are enormous. Use a library, or a simple check plus a confirmation email.
- Avoid catastrophic backtracking. Nested repeats like
(a+)+can take exponential time on bad input (ReDoS). - Add a comment explaining what a long pattern is for. Future readers can’t decode it quickly.