First Patterns

Literals: what you type is what you match

Most characters match themselves. The pattern cat matches the three letters cat wherever they appear:

import re

re.search(r"cat", "the cat sat")   # match! found "cat"
re.search(r"cat", "the dog sat")   # no match

Check (verified):

>>> re.search(r"cat", "the cat sat").group()
'cat'
>>> re.search(r"cat", "the dog sat") is None
True

re.search scans the whole string for the pattern. .group() shows what matched. (r"..." is a raw string — backslashes stay backslashes. Always use raw strings for regex in Python; you'll see why in Escaping.)

Note: cat also matches inside concatenate. Regex doesn't know about words — yet. (Anchors fixes that.)

The dot: "any character"

. matches any single character (except a newline, by default):

>>> re.search(r"c.t", "cat").group()
'cat'
>>> re.search(r"c.t", "cut").group()
'cut'
>>> re.search(r"c.t", "ct") is None   # . needs ONE character
True

All verified. The dot is your first wildcard — "I don't care what goes here, something goes here."

findall: every match, not just the first

>>> re.findall(r"c.t", "cat cut cot cit")
['cat', 'cut', 'cot', 'cit']

findall returns a list of all non-overlapping matches. search finds the first; findall finds them all. You'll use both constantly.

Try it in the shell

echo -e "cat\ncut\ndog" | grep -E "c.t"

Check (verified):

cat
cut

grep -E uses "extended" regex — closest to what this guide teaches. (Plain grep uses an older dialect where + and ? need backslashes. Save yourself the confusion: always -E.)

What "match" means

A pattern doesn't have to match the whole string — it matches a substring unless you say otherwise. re.search(r"cat", "concatenate") matches. This surprises beginners exactly once, then becomes the most useful thing about regex: you're usually hunting for a needle, not validating the haystack.

Next: Character Classes