What is the difference between * and + in regex?

* matches zero or more occurrences (so "ab*c" matches "ac", "abc", "abbc"). + matches one or more occurrences (so "ab+c" matches "abc", "abbc" but NOT "ac"). Use * when the element is optional, + when at least one is required.

What does the ? quantifier do?

The ? makes the preceding element optional (matches zero or one time). For example, "colou?r" matches both "color" and "colour". After another quantifier like * or +, it makes it non-greedy (match as few as possible).

How do capture groups work?

Parentheses () create capture groups that extract matched text. For example, (\d{4})-(\d{2})-(\d{2}) matches a date and captures year, month, and day separately. Use (?:...) for non-capturing groups when you only need grouping.

Regex Cheatsheet

Complete interactive regular expression reference with live testing. Character classes, quantifiers, groups, lookaheads, and common patterns.

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Character Classes

.

Any character except newline

a.c → "abc", "a1c", "a-c"

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\d

Any digit (0-9)

\d{3} → "123", "456"

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\D

Any non-digit

\D+ → "abc", "hello"

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\w

Word character (a-z, A-Z, 0-9, _)

\w+ → "hello_42"

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\W

Non-word character

\W → "!", "@", " "

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\s

Whitespace (space, tab, newline)

a\sb → "a b", "a\tb"

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\S

Non-whitespace

\S+ → "hello", "42"

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[abc]

Any of a, b, or c

[aeiou] → "a", "e", "i"

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[^abc]

Not a, b, or c

[^0-9] → "a", "!"

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[a-z]

Range: a to z

[A-Za-z] → "a", "Z"

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Quantifiers

*

Zero or more

ab*c → "ac", "abc", "abbc"

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+

One or more

ab+c → "abc", "abbc" (not "ac")

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?

Zero or one

colou?r → "color", "colour"

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{n}

Exactly n times

\d{4} → "2024", "1234"

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{n,}

n or more times

\d{2,} → "12", "123", "1234"

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{n,m}

Between n and m times

\d{2,4} → "12", "123", "1234"

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*?

Zero or more (lazy)

<.*?> → "<b>" (not "<b>text</b>")

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+?

One or more (lazy)

\w+? → First character only

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Anchors & Boundaries

^

Start of string/line

^Hello → "Hello world" (start)

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$

End of string/line

world$ → "Hello world" (end)

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\b

Word boundary

\bcat\b → "cat" (not "catch")

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\B

Non-word boundary

\Bcat\B → "concatenate" (inner)

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Groups & References

(abc)

Capturing group

(\d{3})-(\d{4}) → "123-4567"

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(?:abc)

Non-capturing group

(?:ab)+ → "abab"

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(?<name>)

Named group

(?<year>\d{4}) → "2024"

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\1

Back-reference to group 1

(\w)\1 → "aa", "bb"

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(a|b)

Alternation (or)

(cat|dog) → "cat" or "dog"

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Lookahead & Lookbehind

(?=abc)

Positive lookahead

\d(?=px) → "5" in "5px"

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(?!abc)

Negative lookahead

\d(?!px) → "5" in "5em"

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(?<=abc)

Positive lookbehind

(?<=\$)\d+ → "50" in "$50"

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(?<!abc)

Negative lookbehind

(?<!\$)\d+ → "50" in "50"

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Flags

g

Global - match all occurrences

/cat/g → All "cat" matches

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i

Case-insensitive

/hello/i → "Hello", "HELLO"

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m

Multiline - ^ and $ match per line

/^start/m → Start of each line

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s

Dotall - . matches newlines

/a.b/s → "a\nb"

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Common Patterns

Email

Basic email validation

[\w.-]+@[\w.-]+\.\w{2,} → "user@site.com"

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URL

Basic URL matching

https?://[\w./%-]+ → "https://site.com/path"

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IP Address

IPv4 address

\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3} → "192.168.1.1"

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Phone

US phone number

\d{3}[-.\s]?\d{3}[-.\s]?\d{4} → "555-123-4567"

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Hex Color

CSS hex color

#[0-9a-fA-F]{3,6} → "#fff", "#FF0000"

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Date

YYYY-MM-DD format

\d{4}-\d{2}-\d{2} → "2024-01-15"

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Frequently Asked Questions

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