Python Regular Expressions (regex)

1Python RegEx: overview

A regular expression (regex) is a short pattern that describes the shape of some text — such as "three digits, a dash, then four digits" — which Python's re module uses to find, extract, check and replace text.

Before this lesson: Python Strings, Python Lists

2Python RegEx: diagram

Quick reference: Python RegEx

• Python Regex Cheat Sheet (re Module Quick Reference) — Python regular expressions live in the `re` module: write the pattern as a raw string, then use `re.search()`…
• How to Remove Whitespace from a String in Python — Use `text.strip()` to remove whitespace from both ends, or `"".join(text.split())` to remove it everywhere, i…
• How to Split a String into Words in Python — Call `text.split()` with no arguments: it splits on any whitespace and returns a list of words, such as `"to …

4Python RegEx: syntax

import re

re.search(r"pattern", text)       # first match -> Match object or None
re.findall(r"pattern", text)      # list of every match
re.fullmatch(r"pattern", text)    # the WHOLE text must match
re.sub(r"pattern", "new", text)   # replace every match
m.group()   m.group(1)            # whole match, first ( ) group

\d digit   \w word char   \s whitespace   . any char   \b word edge
+ one or more   * zero or more   ? optional   {n} exactly n   [abc] one of

5Python RegEx: run the example

Press Run (or Ctrl + Enter). The output (step 6) appears directly below the code. Change anything and run it again.

import re

message = "Call 555-0142 or 555-0199. Orders #A1042 and #B2077 shipped 2024-05-17."

phones = re.findall(r"\d{3}-\d{4}", message)
print("Phones:", phones)

order_ids = re.findall(r"#([A-Z]\d+)", message)
print("Order IDs:", order_ids)

m = re.search(r"(\d{4})-(\d{2})-(\d{2})", message)
if m:
    year, month, day = m.groups()
    print(f"Shipped: {day}/{month}/{year}")

print(re.sub(r"\d{3}-(\d{4})", r"***-\1", message))

for code in ["A1042", "a1042", "B20"]:
    print(code, bool(re.fullmatch(r"[A-Z]\d{4}", code)))

Expected outputPhones: ['555-0142', '555-0199']
Order IDs: ['A1042', 'B2077']
Shipped: 17/05/2024
Call ***-0142 or ***-0199. Orders #A1042 and #B2077 shipped 2024-05-17.
A1042 True
a1042 False
B20 False

7Python RegEx: how the example works

Hover or tap a step to highlight its lines in the example above.

1. Line 1re is Python's built-in regular expression module.
2. Line 3A message containing phone numbers, order IDs and a date, all mixed into ordinary text.
3. Line 5-6\d{3}-\d{4} means three digits, a dash, four digits. findall() returns every match as a list of strings.
4. Line 8-9The # must appear literally; the group ([A-Z]\d+) captures one capital letter followed by digits. Because the pattern has a group, findall() returns just the group — without the #.
5. Line 11search() finds the first date. The three groups capture year, month and day separately.
6. Line 12-14if m: guards against None (no match). m.groups() returns all groups as a tuple, which is unpacked and printed in day/month/year order.
7. Line 16re.sub() replaces every phone number. \1 in the replacement puts back group 1 — the last four digits — so the numbers are masked, not deleted.
8. Line 18-19fullmatch() checks the whole string: one capital letter and exactly four digits. a1042 fails on the lowercase letter and B20 is too short.

8Python RegEx: common mistakes

import re
print(re.findall("\bcat\b", "cat catalog cat"))

import re
print(re.findall(r"\bcat\b", "cat catalog cat"))

import re
m = re.search(r"\d+", "no numbers here")
print(m.group())

import re
m = re.search(r"\d+", "no numbers here")
if m:
    print(m.group())
else:
    print("No number found")

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

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

9Python RegEx: exercises

Write your answer, then press Check. The checks run in your browser.

import re

def extract_numbers(text):
    # find every run of digits and return them as ints
    pass

print(extract_numbers("I bought 3 apples for 120 rupees"))

import re

def extract_numbers(text):
    return [int(n) for n in re.findall(r"\d+", text)]

print(extract_numbers("I bought 3 apples for 120 rupees"))

import re

text = "cat cart scat car"
print(re.findall(r"ca.", text))
print(re.findall(r"\bca\w*", text))
print(re.sub(r"a", "o", "banana", count=2))

['cat', 'car', 'cat', 'car']
['cat', 'cart', 'car']
bonona

2 more exercises → 4 more examples

Further reading

• json
• csv
• re
• sqlite3
• logging
• unittest
• Requests documentation
• Installing packages

10Next lesson

Related:
Python StringsWorking with JSON in PythonPython CSV Files

← Python CSV Files · All Practical Python lessons · Practise in the IDE