Learn AI with Python · Lesson

re Module: search, match, findall, sub

re.search(), re.match(), re.findall(), re.sub(), re.compile() for performance.

re Module: search, match, findall, sub is a free Learn AI with Python lesson on CoddyKit — lesson 2 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Learn AI with Python learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

The re Module

Python exposes regex through the built-in re module. The core functions are search, match, findall, and sub.

Knowing which to reach for is half the battle.

import re   # always import it first

re.search — Find Anywhere

re.search(pattern, text) scans the whole string for the first match and returns a Match object, or None if nothing matches.

import re

m = re.search("\d+", "order 42 shipped")
if m:
    print(m.group())   # "42"
    print(m.start())   # 6

re.match — Anchored at the Start

re.match only matches at the beginning of the string. If the pattern is not at position 0, it returns None even if it appears later.

import re

print(re.match("\d+", "42 items"))    # matches "42"
print(re.match("\d+", "items 42"))    # None

search vs match

The key difference:

match — pattern must start at index 0
search — pattern may appear anywhere

Most of the time you want search. Use match only when you specifically need a prefix check.

re.findall — Get Every Match

re.findall returns a list of all non-overlapping matches as strings (not Match objects). Ideal for extraction tasks.

import re

emails = re.findall("\w+@\w+\.\w+", "a@x.com and b@y.org")
print(emails)   # ["a@x.com", "b@y.org"]

findall with Groups

If your pattern has capturing groups, findall returns the captured parts instead of the whole match — a common surprise.

Here each tuple holds the area code and the rest.

import re

print(re.findall("(\d{3})-(\d{4})", "555-1234 and 999-8765"))
# [("555", "1234"), ("999", "8765")]

re.finditer — Match Objects

When you need positions or groups for every match, use re.finditer. It yields Match objects you can iterate over lazily.

import re

for m in re.finditer("\d+", "a1 b22 c333"):
    print(m.group(), "at", m.start())

re.sub — Search and Replace

re.sub(pattern, replacement, text) replaces every match with the replacement string and returns a new string.

Foundational for text cleaning — masking, normalizing, stripping.

import re

clean = re.sub("\d+", "#", "Room 101 and 202")
print(clean)   # "Room # and #"

sub with Backreferences and Functions

The replacement can reference captured groups with \1, or be a function for dynamic logic.

import re

# Swap "first last" -> "last, first"
print(re.sub("(\w+) (\w+)", "\\2, \\1", "Ada Lovelace"))

# Function replacement: double each number
print(re.sub("\d+", lambda m: str(int(m.group()) * 2), "a1 b2"))

re.compile — Reuse Patterns

When you use a pattern many times (e.g. in a loop), re.compile builds it once into a reusable pattern object. This is faster and reads cleanly.

import re

num = re.compile("\d+")
for line in ["a1", "b22", "c333"]:
    print(num.findall(line))

Useful Flags

Flags change matching behavior:

re.IGNORECASE — case-insensitive
re.MULTILINE — ^/$ match each line
re.DOTALL — . also matches newlines

import re

print(re.findall("cat", "Cat CAT cat", re.IGNORECASE))
# ["Cat", "CAT", "cat"]

Quick Check: search vs findall

You need a list of every phone number that appears anywhere in a document.

Recap: The re Module

You now know the core regex functions:

re.search — first match anywhere (Match object)
re.match — match only at the start
re.findall — list of all matches (groups change the output)
re.finditer — Match objects with positions
re.sub — replace matches, with backrefs or a function
re.compile and flags for reuse and control

Next: capturing groups and named groups.

Frequently asked questions

Is the “re Module: search, match, findall, sub” lesson free?

Yes — the full text of “re Module: search, match, findall, sub” is free to read here on the web, and the Learn AI with Python course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Learn AI with Python course, upgrade to CoddyKit PRO.

What will I learn in “re Module: search, match, findall, sub”?

re.search(), re.match(), re.findall(), re.sub(), re.compile() for performance. You practise Learn AI with Python with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Learn AI with Python?

No prior experience is required. Learn AI with Python on CoddyKit is structured for beginners through advanced learners; this is — lesson 2 of 4, so you can start here or from the beginning and move at your own pace.

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