The asterisk in regular expressions

What * repeats

The * applies only to the single item right before it: one character, one character class, or one group in parentheses.

Pattern	Matches	Doesn't match
a*	an empty string, a, aaaa	b as a whole
ab*	a, ab, abbb	abab as a whole
(ab)*	an empty string, ab, abab	aab as a whole
[0-9]*	an empty string, 7, 2026	20x6 as a whole
.*	anything on one line	a line break, unless the "dot matches newline" flag is on

The "zero" catches people out

Because * allows zero repeats, a pattern built from it can match nothing at all, and it happily does:

import re

print(re.findall(r"\d*", "a1"))   # ['', '1', '']
print(bool(re.fullmatch(r"\d*", "")))   # True: an empty string is "zero digits"

If you need at least one, use + instead: \d+. A form field validated with ^\d*$ accepts an empty value, which is rarely what anyone meant.

Greedy and lazy: * versus *?

By default * is greedy: it grabs as much as it can and only gives back what the rest of the pattern needs. Adding a ? makes it lazy, so it takes as little as possible.

const html = "<b>bold</b>";
console.log(html.match(/<.*>/g));   // [ '<b>bold</b>' ]
console.log(html.match(/<.*?>/g));  // [ '<b>', '</b>' ]

Java, PCRE, .NET and Python 3.11 and later also support a possessive form, *+, which never gives anything back. It's useful for stopping runaway backtracking on long inputs.

Matching a literal asterisk

Where	Write	Notes
Any regex	\*	The backslash makes it an ordinary character
Any regex	[*]	Inside square brackets, * is already literal
Python	r"\*"	Use a raw string so Python leaves the backslash alone
JavaScript literal	/\*/
JavaScript string	new RegExp("\\*")	The string needs a doubled backslash
Escaping user input	re.escape(s) in Python, RegExp.escape(s) in JavaScript	Escapes * and every other special character

RegExp.escape is new: it arrived with ES2025 and works in Node 24 and current browsers. In older environments, s.replace(/[.*+?^${}()|[\]\\]/g, "\\$&") does the same job.

grep and sed

In grep, sed and other POSIX tools, * means the same thing in both basic and extended syntax, and \* still matches a real asterisk. One quirk of basic syntax: a * at the very start of a pattern has nothing to repeat, so it's treated as a literal. grep '*note' file.txt finds "*note", while grep -E '*note' file.txt stops with an error. Don't rely on either. Write grep '\*note' and the intent is clear to everyone, including other regex engines.

Quote the pattern in the shell, or the shell may expand the * into file names before grep ever sees it. See the asterisk in Bash.

Regex * is not a glob *

In a file pattern like *.txt, * means "any characters". In a regex it means "repeat the previous thing", and a pattern can't start with it:

import re
re.compile("*.txt")   # re.error: nothing to repeat at position 0

The regex version of the glob *.txt is ^.*\.txt$. The glob * is the regex .*, and the glob ? is the regex ..

Where the name comes from

The mathematician Stephen Kleene described this operator in the 1950s while working on what became regular languages. It's still called the Kleene star, and in theory books Σ* means "every string you can make from the alphabet Σ", including the empty one.