Regular Expression - Basic Syntax

Basic Syntax

Literal Characters

Match exact characters:

cat # Matches "cat"
hello # Matches "hello"

Character Classes

Pattern	Matches
[abc]	Any single character: a, b, or c
[^abc]	Any character EXCEPT a, b, or c
[a-z]	Any lowercase letter
[A-Z]	Any uppercase letter
[0-9]	Any digit
[a-zA-Z]	Any letter (upper or lower)
[a-zA-Z0-9]	Any letter or digit

Predefined Character Classes

Pattern	Equivalent	Matches
\d	[0-9]	Any digit
\D	[^0-9]	Any non-digit
\w	[a-zA-Z0-9_]	Any word character (letter, digit, underscore)
\W	[^a-zA-Z0-9_]	Any non-word character
\s	[ \t\n\r\f\v]	Any whitespace (space, tab, newline)
\S	[^ \t\n\r\f\v]	Any non-whitespace
.	-	Any character except newline

Quantifiers

Pattern	Matches
*	0 or more times
+	1 or more times
?	0 or 1 time (optional)
{n}	Exactly n times
{n,}	n or more times
{n,m}	Between n and m times

Examples:

\d+ # One or more digits: "123", "5"
\d{3} # Exactly 3 digits: "123", "456"
\d{2,4} # 2 to 4 digits: "12", "123", "1234"
colou?r # "color" or "colour"
https? # "http" or "https"

Anchors

Pattern	Matches
^	Start of string/line
$	End of string/line
\b	Word boundary
\B	Not a word boundary

Examples:

^hello # "hello" at start: "hello world" ✓, "say hello" ✗
world$ # "world" at end: "hello world" ✓, "world peace" ✗
\bcat\b # "cat" as whole word: "cat" ✓, "category" ✗

Grouping and Alternation

Pattern	Meaning
(...)	Capturing group
(?:...)	Non-capturing group
`	`

Examples:

(cat|dog) # Matches "cat" OR "dog"
gr(a|e)y # Matches "gray" OR "grey"
(\d{3})-\d{4} # Matches "123-4567" (captures "123")