Basic Characters

Pattern	Description	Example Pattern	Matched String
`^`	Start of string (each line)	`{regex} ^Hello`	”Hello World”
`$`	End of string (each line)	`{regex} World$`	”Hello World”
`.`	Any character (except newline)	`{regex} a.c`	”abc”, “adc”
`\|`	Or	`{regex} cat\|dog`	”cat”, “dog”
`\`	Escape special characters	`{regex} a\.b`	”a.b”

Quantifiers

Pattern	Description	Example Pattern	Matched String
`*`	Zero or more occurrences	`{regex} ab*c`	”ac”, “abc”, “abbc”
`+`	One or more occurrences	`{regex} ab+c`	”abc”, “abbc”
`?`	Zero or one occurrence	`{regex} ab?c`	”ac”, “abc”
`{n}`	Exactly n occurrences	`{regex} a{3}`	”aaa”
`{n,}`	n or more occurrences	`{regex} a{2,}`	”aa”, “aaa”, “aaaa”
`{,m}`	Up to m occurrences	`{regex} a{,3}`	”a”, “aa”, “aaa”
`{n,m}`	Between n and m occurrences	`{regex} a{1,3}`	”a”, “aa”, “aaa”

Character Classes

Pattern	Description	Example Pattern	Matched String
`\d`	Digit (0-9)	`{regex} \d\d\d`	”123”
`\D`	Not a digit	`{regex} \D\D`	”AB”
`\w`	Word character (a-zA-Z0-9_)	`{regex} \w+`	”HelloWorld”
`\W`	Not a word character	`{regex} \W+`	”!@#$“
`\s`	Whitespace (space, tab, newline)	`{regex} \s+`	” \t\n”
`\S`	Not whitespace	`{regex} \S+`	”NoSpace”
`\b`	Word boundary	`{regex} \bcat\b`	”The cat sat”
`\B`	Not a word boundary	`{regex} \Ban`	”banana”
`\A`	(Absolute) start of the string	`{regex} \AHello`	”Hello World”
`\Z`	(Absolute) end of the string (before newline)	`{regex} World\Z`	”Hello World”

Sets

Pattern	Description	Example Pattern	Matched String
`[abc]`	Any character in the set	`{regex} b[ae]d`	”bad”, “bed”
`[^abc]`	Any character not in the set	`{regex} [^xyz]+`	”uvw”
`[a-z]`	Range of characters	`{regex} [a-f]+`	”abcdef”

Groups

Pattern	Description	Example Pattern	Matched String
`()`	Grouping and capturing	`{regex} (ab)+`	”abab”
`(?#)`	Comment; ignored by the engine	`{regex} a(?#this is a comment)b`	”ab”
`(?P<name>A)`	Named capturing group	`{regex} (?P<word>\w+) \g<word>`	”test test”
`(?:A)`	Non-capturing group	`{regex} (?:ab)+c`	”ababc”
`(?P=name)`	Backreference to a named group	`{regex} (?P<char>.)(?P=char)`	”aa”, “bb”

Assertions

Pattern	Description	Example Pattern	Matched String
`A(?=B)`	Positive lookahead: A followed by B	`{regex} Hello(?= World)`	”Hello” in “Hello World”
`A(?!B)`	Negative lookahead: A not followed by B	`{regex} Hello(?! World)`	”Hello” in “Hello There”
`(?<=B)A`	Positive lookbehind: A preceded by B	`{regex} (?<=Hello)World`	”World” in “Hello World”
`(?<!B)A`	Negative lookbehind: A not preceded by B	`{regex} (?<!Goodbye)World`	”World” in “Hello World”

Flags (Modifiers)

Flag	Common Name	Description
`i`	Case-Insensitive	Matches regardless of case (e.g., `a` matches `A`).
`g`	Global	Finds all matches, not just the first.
`m`	Multiline	`^` and `$` match the start/end of lines (after/before newlines).
`s`	Dotall	`.` matches any character, including newline.
`u`	Unicode	Enables full Unicode support for character classes (`\w`, `\d`, `\s`, etc.).