Regular Expressions

Basic Syntax

`/.../`	Start and end regex delimiters
`\|`	Alternation
`()`	Grouping

Position Matching

`^`	Start of string or start of line in multi-line mode
`\A`	Start of string
`$`	End of string or end of line in multi-line mode
`\Z`	End of string
`\b`	Word boundary
`\B`	Not word boundary
`\<`	Start of word
`\>`	End of word

Character Classes

`\s`	Whitespace
`\S`	Not whitespace
`\w`	Word
`\W`	Not word
`\d`	Digit
`\D`	Not digit
`\x`	Hexadecimal digit
`\O`	Octal digit

Special Characters

`\n`	Newline
`\r`	Carriage return
`\t`	Tab
`\v`	Vertical tab
`\f`	Form feed
`\xxx`	Octal character xxx
`\xhh`	Hex character hh

Groups and Ranges

`.`	Any character except newline (\n)
`(a\|b)`	a or b
`(...)`	Group
`(?:...)`	Passive (non-capturing) group
`[abc]`	a, b or c
`[^abc]`	Not a, b or c
`[a-z]`	Letters from a to z
`[A-Z]`	Uppercase letters from A to Z
`[0-9]`	Digits from 0 to 9
Note: Ranges are inclusive.

Quantifiers

`*`	0 or more
`+`	1 or more
`?`	0 or 1
`{3}`	Exactly 3
`{3,}`	3 or more
`{3,5}`	3, 4 or 5
Note: Quantifiers are greedy - they match as many times as possible. Add a `?` after the quantifier to make it ungreedy.

Escape Sequences

`\`	Escape following character Used to escape any of the following metacharacters: `{}[]()^$.\|*+?\`.
`\Q`	Begin literal sequence
`\E`	End literal sequence

String Replacement

`$1`	1st group
`$2`	2nd group
`$n`	nth group
$`	Before matched string
`$'`	After matched string
`$+`	Last matched string
`$&`	Entire matched string
Note: Some regex implementations use `\` instead of `$`.

Assertions

`(?=…)`	Positive lookahead
`(?!…)`	Negative lookahead
`(?<=…)`	Positive lookbehind
`(?<!…)`	Negative lookbehind
`(?>…)`	Atomic (once-only) group
`(?(a)b)`	Conditional (if a then match b)
`(?(a)b\|c)`	Conditional (if a then match b, else match c)
`(?#…)`	Comment

POSIX

`[:upper:]`	Uppercase letters
`[:lower:]`	Lowercase letters
`[:alpha:]`	All letters
`[:alnum:]`	Digits and letters
`[:digit:]`	Digits
`[:xdigit:]`	Hexadecimal digits
`[:punct:]`	Punctuation
`[:blank:]`	Space and tab
`[:space:]`	Blank characters
`[:cntrl:]`	Control characters
`[:graph:]`	Printed characters
`[:print:]`	Printed characters and spaces
`[:word:]`	Digits, letters and underscore

Pattern Modifiers

`g`	Global match
`i`	Case-insensitive
`m`	Multi-line mode Causes `^` and `$` to also match the start/end of lines.
`s`	Single-line mode. Causes `.` to match all, including line breaks.
`x`	Allow comments and whitespace in pattern
`e`	Evaluate replacement
`U`	Ungreedy mode