Q: What is the difference between greedy and lazy matching in regex?

Greedy matching (default) matches the longest possible string. Lazy matching (add ?) matches the shortest possible string. For example, " " matches the entire " text ", but " " matches only " ".

Question 1

What is the difference between * and + in regex?

Accepted Answer

* (asterisk) means the preceding pattern repeats 0 or more times. + (plus) means the preceding pattern must repeat 1 or more times. For example, a* matches empty strings, but a+ requires at least one "a" to match.

Question 2

What is a capturing group?

Accepted Answer

A capturing group is a part of a regex enclosed in parentheses () that allows you to extract the matched text separately. For example, (\d{4})-(\d{2})-(\d{2}) matches a date and captures year, month, and day as separate groups. (?:...) is a non-capturing group.

Question 3

What are lookahead and lookbehind?

Accepted Answer

Lookahead and lookbehind are assertions that check for patterns without consuming characters. (?=abc) is a positive lookahead that checks if abc follows, and (?<=abc) is a positive lookbehind that checks if abc precedes. Negative versions use (?!) and (?<!).

Question 4

What is the difference between greedy and lazy matching in regex?

Accepted Answer

Greedy matching (default) matches the longest possible string. Lazy matching (add ?) matches the shortest possible string. For example, "<.*>" matches the entire "

text

", but "<.*?>" matches only "

".

Pattern	Description	Example
`^`	Start of string	^Hello
`$`	End of string	world$
`\b`	Word boundary	\bword\b
`\B`	Not word boundary	\Bword\B

Pattern	Description	Example
`.`	Any character except newline	-
`\d`	Digit [0-9]	-
`\D`	Not a digit	-
`\w`	Word character	-
`\W`	Not a word character	-
`\s`	Whitespace	-
`\S`	Not whitespace	-
`[abc]`	a, b, or c	-
`[^abc]`	Not a, b, or c	-
`[a-z]`	a to z	-

Pattern	Description	Example
`*`	0 or more	a*
`+`	1 or more	a+
`?`	0 or 1	a?
`{n}`	Exactly n	a{3}
`{n,}`	n or more	a{2,}
`{n,m}`	Between n and m	a{2,4}
`*?`	Lazy 0 or more	-
`+?`	Lazy 1 or more	-

Pattern	Description	Example
`(abc)`	Capturing group	-
`(?:abc)`	Non-capturing group	-
`(?<name>abc)`	Named group	-
`\1`	Backreference	-
`(a\|b)`	a or b	-

Pattern	Description	Example
`(?=abc)`	Positive lookahead	-
`(?!abc)`	Negative lookahead	-
`(?<=abc)`	Positive lookbehind	-
`(?<!abc)`	Negative lookbehind	-

Regex Cheatsheet

#Anchors

#Character Classes

#Quantifiers

#Groups & References

#Lookaround

#Flags

#Common Patterns

#Frequently Asked Questions

Pattern	Description	Example
`g`	Global search	-
`i`	Case insensitive	-
`m`	Multiline mode	-
`s`	Dotall mode	-
`u`	Unicode mode	-

Pattern	Description	Example
`^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$`	Email	-
`^https?:\/\/[\S]+$`	URL	-
`^\d{3}-\d{4}-\d{4}$`	Phone (Korea)	-
`^\d{4}-\d{2}-\d{2}$`	Date (YYYY-MM-DD)	-
`^#?([a-fA-F0-9]{6}\|[a-fA-F0-9]{3})$`	Hex color	-