Quantifiers

A quantifier says how many of the previous thing: "three digits," "one or more letters," "optional."

The big four

Quantifier	Means	Example	Matches
*	Zero or more	a*	`,a,aaa`
+	One or more	a+	a, aaa (not empty)
?	Zero or one (optional)	colou?r	color, colour
{n,m}	Between n and m	\d{2,4}	12, 123, 1234

>>> re.findall(r"\d+", "abc123def45")
['123', '45']                    # one or more digits, greedily
>>> re.findall(r"colou?r", "color colour")
['color', 'colour']              # the u is optional
>>> re.findall(r"\d{2,4}", "1 12 123 12345")
['12', '123', '1234']            # 2-4 digits; "12345" gives "1234"

All verified. {n} is exactly n; {n,} is n or more. The precise forms ({4}, {2,4}) are your friends for fixed-format data like dates and zip codes.

Greedy vs. lazy: the essential gotcha

By default, quantifiers are greedy — they grab as much as possible:

>>> re.search(r"<.*>", "<b>hi</b>").group()
'<b>hi</b>'     # grabbed EVERYTHING between first < and last >

You wanted <b>, you got the whole tag pair. The fix: lazy quantifiers (*?, +?) grab as little as possible:

>>> re.search(r"<.*?>", "<b>hi</b>").group()
'<b>'           # stops at the first >

Both verified. The rule: when matching between delimiters (tags, quotes, brackets), reach for the lazy version first. Greedy is right when you want "everything up to the end" — lazy is right when you want "the smallest piece."

>>> re.findall(r'".*?"', 'say "hi" and "bye"')
['"hi"', '"bye"']     # lazy: two separate quoted strings
>>> re.findall(r'".*"', 'say "hi" and "bye"')
['"hi" and "bye"']    # greedy: one giant match

Verified. This pair of lines is worth staring at until it clicks — it's the single most common regex bug in the wild.

Common shapes

Shape	Use
\d+	A number
\w+	A word / identifier
".*?"	A quoted string
\d{4}-\d{2}-\d{2}	An ISO date
https?://	http:// or https://
\s*	Optional whitespace (the polite glue between tokens)

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