Character Classes

A character class matches one character from a set. It's the answer to "a digit here, I don't care which one."

Brackets: your own sets

>>> re.findall(r"c[au]t", "cat cut cot")
['cat', 'cut']          # [au] = "a or u"; "cot" doesn't match
>>> re.findall(r"[a-z]+", "hello 123")
['hello']               # [a-z] = any lowercase letter
>>> re.findall(r"[a-zA-Z0-9]+", "Hello 123!")
['Hello', '123']        # letters and digits (+ explained next chapter)

Inside brackets, most special characters lose their powers: [.] matches a literal dot, [*] a literal star. The exceptions: ^ at the start negates, and - defines ranges:

>>> re.findall(r"[^0-9]+", "abc123def")
['abc', 'def']          # [^...] = "anything EXCEPT these"
>>> re.findall(r"[0-9-]+", "2026-10-13")
['2026-10-13']         # - at the start/end is literal

All verified. ([^0-9] is "not a digit" — the ^ only negates when it's first inside the brackets. Anywhere else, it's a literal caret.)

Shorthands: the ones you'll use daily

Shorthand	Means	Same as
`\d`	A digit	`[0-9]`
`\D`	Not a digit	`[^0-9]`
`\w`	Word character (letters, digits, `_`)	`[a-zA-Z0-9_]`
`\W`	Not a word character	`[^a-zA-Z0-9_]`
`\s`	Whitespace (space, tab, newline)	`[ \t\n\r\f\v]`
`\S`	Not whitespace

(Capital = opposite. That's the whole mnemonic.)

>>> re.findall(r"\d+", "order 42, qty 7")
['42', '7']
>>> re.findall(r"\w+", "hello_world 123")
['hello_world', '123']
>>> re.split(r"\s+", "a  b\tc")
['a', 'b', 'c']

All verified. Note \w includes underscore and digits — "word character" is really "identifier character," which is what you want when grabbing variable names and usernames.

Careful: \d matches only ASCII 0-9 in most flavors, but in Python 3 it matches Unicode digits too (like ٣). For log parsing it never matters; for strict validation, use [0-9] to be explicit.

Putting it together: a date

>>> re.search(r"\d{4}-\d{2}-\d{2}", "meeting on 2026-10-13 at noon").group()
'2026-10-13'

Verified. Read it aloud: "four digits, dash, two digits, dash, two digits." You're already reading regex. ({4} is a quantifier — next chapter.)

Next: Quantifiers