Question 1

What is a regular expression (regex)?

Accepted Answer

A regular expression (regex) is a sequence of characters that defines a search pattern. Regex is used for pattern matching within strings — finding, replacing, or validating text. It is supported in virtually every programming language and many text editors.

Question 2

What does the g flag mean in regex?

Accepted Answer

The g (global) flag tells the regex engine to find all matches in the string, not just the first one. Without the g flag, the regex stops after the first match. Other common flags include i (case-insensitive), m (multiline), s (dotAll), and u (unicode).

Question 3

How do I match an email address with regex?

Accepted Answer

A common email regex pattern is: /[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+.[a-zA-Z]{2,}/. However, email validation with regex is notoriously complex — the full RFC 5322 specification is extremely long. For production use, consider using a dedicated email validation library.

Question 4

What is the difference between * and + in regex?

Accepted Answer

The * quantifier matches zero or more occurrences of the preceding element, while + matches one or more occurrences. For example, /a*/ matches empty string, a, aa, aaa, etc., while /a+/ matches a, aa, aaa but NOT an empty string.

Question 5

How do I escape special characters in regex?

Accepted Answer

Special regex characters (. * + ? ^ $ { } [ ] ( ) | ) must be escaped with a backslash () to match them literally. For example, to match a period, use . instead of just . (which matches any character). In JavaScript strings, you need double backslashes: \.

Question 6

What are capture groups in regex?

Accepted Answer

Capture groups are created with parentheses () and allow you to extract specific parts of a match. For example, /(d{4})-(d{2})-(d{2})/ captures year, month, and day separately from a date string. Non-capturing groups (?:...) group without capturing.

Pattern	Description	Example
.	Any character except newline	h.t matches hat, hot, hit
\d	Any digit (0-9)	\d+ matches 123, 42
\D	Any non-digit	\D+ matches abc, hello
\w	Word character (a-z, A-Z, 0-9, _)	\w+ matches hello_world
\W	Non-word character	\W matches @, #, spaces
\s	Whitespace (space, tab, newline)	\s+ matches spaces/tabs
\S	Non-whitespace	\S+ matches any_word
\b	Word boundary	\bcat\b matches cat not cats

Pattern	Description	Example
*	Zero or more	ab*c matches ac, abc, abbc
+	One or more	ab+c matches abc, abbc (not ac)
?	Zero or one (optional)	colou?r matches color, colour
{n}	Exactly n times	\d{3} matches 123
{n,}	n or more times	\d{2,} matches 12, 123, 1234
{n,m}	Between n and m times	\d{2,4} matches 12, 123, 1234

Pattern	Description	Example
[abc]	Any of a, b, or c	[aeiou] matches vowels
[^abc]	Not a, b, or c	[^0-9] matches non-digits
[a-z]	Range: a through z	[a-zA-Z] matches any letter
(abc)	Capture group	(\d+)-(\d+) captures both numbers
(?:abc)	Non-capturing group	(?:http\|https):// groups without capturing
a\|b	Alternation (or)	cat\|dog matches cat or dog

Pattern	Description
^	Start of string (or line with m flag)
$	End of string (or line with m flag)
\b	Word boundary
\B	Non-word boundary

Pattern	Description	Example
(?=...)	Positive lookahead	\d+(?=px) matches 10 in 10px
(?!...)	Negative lookahead	\d+(?!px) matches 10 in 10em
(?<=...)	Positive lookbehind	(?<=\$)\d+ matches 50 in $50
(?<!...)	Negative lookbehind	(?<!\$)\d+ matches 50 in €50

Regex Cheat Sheet: Complete Regular Expression Reference

Introduction to Regular Expressions

Basic Character Matching

Quantifiers

Character Classes & Groups

Anchors & Boundaries

Lookaheads & Lookbehinds

Common Regex Patterns

Regex Flags

Test Your Regex Patterns

Flag	Name	Description
g	Global	Find all matches, not just the first
i	Case-insensitive	Match regardless of case
m	Multiline	^ and $ match line starts/ends
s	DotAll	. matches newline characters too
u	Unicode	Enable full Unicode matching