Regular Expression Language Elements

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A regular expression is a pattern that the regular expression engine attempts to match in input text. A pattern consists of one or more character literals, operators, or constructs. For a brief introduction, see .NET Framework Regular Expressions.

Each section in this quick reference lists a particular category of characters, operators, and constructs that you can use to define regular expressions:

• Character escapes

• Character classes

• Anchors

• Grouping constructs

• Quantifiers

• Backreference constructs

• Alternation constructs

• Substitutions

• Miscellaneous constructs

Character Escapes

The backslash character (\) in a regular expression indicates that the character that follows it either is a special character (as shown in the following table), or should be interpreted literally. For more information, see Character Escapes.

 

Escaped character	Description	Pattern	Matches
\a	Matches a bell character, \u0007.	\a	"\u0007" in "Error!" + '\u0007'
\b	In a character class, matches a backspace, \u0008.	[\b]{3,}	"\b\b\b\b" in "\b\b\b\b"
\t	Matches a tab, \u0009.	(\w+)\t	"item1\t", "item2\t" in "item1\titem2\t"
\r	Matches a carriage return, \u000D. (\r is not equivalent to the newline character, \n.)	\r\n(\w+)	"\r\nThese" in "\r\nThese are\ntwo lines."
\v	Matches a vertical tab, \u000B.	[\v]{2,}	"\v\v\v" in "\v\v\v"
\f	Matches a form feed, \u000C.	[\f]{2,}	"\f\f\f" in "\f\f\f"
\n	Matches a new line, \u000A.	\r\n(\w+)	"\r\nThese" in "\r\nThese are\ntwo lines."
\e	Matches an escape, \u001B.	\e	"\x001B" in "\x001B"
\ nnn	Uses octal representation to specify a character (nnn consists of up to three digits).	\w\040\w	"a b", "c d" in "a bc d"
\x nn	Uses hexadecimal representation to specify a character (nn consists of exactly two digits).	\w\x20\w	"a b", "c d" in "a bc d"
\c X \c x	Matches the ASCII control character that is specified by X or x, where X or x is the letter of the control character.	\cC	"\x0003" in "\x0003" (Ctrl-C)
\u nnnn	Matches a Unicode character by using hexadecimal representation (exactly four digits, as represented by nnnn).	\w\u0020\w	"a b", "c d" in "a bc d"
\	When followed by a character that is not recognized as an escaped character in this and other tables in this topic, matches that character. For example, \* is the same as \x2A. This allows the regular expression engine to disambiguate language elements (such as * or ?) and character literals (represented by \* or \?).	\d+[\+-x\*]\d+\d+[\+-x\*\d+	"2+2" and "3*9" in "(2+2) * 3*9"

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Character Classes

A character class matches any one of a set of characters. Character classes include the language elements listed in the following table. For more information, see Character Classes.

 

Character class	Description	Pattern	Matches
[ character_group ]	Matches any single character in character_group. By default, the match is case-sensitive.	[ae]	"a" in "gray" "a", "e" in "lane"
[^ character_group ]	Negation: Matches any single character that is not in character_group. By default, characters in character_group are case-sensitive.	[^aei]	"r", "g", "n" in "reign"
[ first - last ]	Character range: Matches any single character in the range from first to last.	[A-Z]	"A", "B" in "AB123"
.	Wildcard: Matches any single character except \n.	a.e	"ave" in "nave" "ate" in "water"
\p{ name }	Matches any single character in the Unicode general category or named block specified by name.	\p{Lu} \p{IsCyrillic}	"C", "L" in "City Lights" "Д", "Ж" in "ДЖem"
\P{ name }	Matches any single character that is not in the Unicode general category or named block specified by name.	\P{Lu} \P{IsCyrillic}	"i", "t", "y" in "City" "e", "m" in "ДЖem"
\w	Matches any word character.	\w	"I", "D", "A", "1", "3" in "ID A1.3"
\W	Matches any non-word character.	\W	" ", "." in "ID A1.3"
\s	Matches any white-space character.	\w\s	"D " in "ID A1.3"
\S	Matches any non-white-space character.	\s\S	" _" in "int __ctr"
\d	Matches any decimal digit.	\d	"4" in "4 = IV"
\D	Matches any character that is not a decimal digit.	\D	" ", "=", " ", "I", "V" in "4 = IV"

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Anchors

Anchors, or atomic zero-width assertions, cause a match to succeed or fail depending on the current position in the string, but they do not cause the engine to advance through the string or consume characters. The metacharacters listed in the following table are anchors. For more information, see Anchors in Regular Expressions.

 

Assertion	Description	Pattern	Matches
^	The match must start at the beginning of the string or line.	^\d{3}	"901-" in "901-333-"
$	The match must occur at the end of the string or before \n at the end of the line or string.	-\d{3}$	"-333" in "-901-333"
\A	The match must occur at the start of the string.	\A\d{3}	"901" in "901-333-"
\Z	The match must occur at the end of the string or before \n at the end of the string.	-\d{3}\Z	"-333" in "-901-333"
\z	The match must occur at the end of the string.	-\d{3}\z	"-333" in "-901-333"
\G	The match must occur at the point where the previous match ended.	\G\(\d\)	"(1)", "(3)", "(5)" in "(1)(3)(5)[7](9)"
\b	The match must occur on a boundary between a \w (alphanumeric) and a \W (nonalphanumeric) character.	\b\w+\s\w+\b	"them them" in "them theme them them"
\B	The match must not occur on a \b boundary.	\Bend\w*\b	"ends", "ender" in "end sends endure lender"

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Grouping Constructs

Grouping constructs delineate subexpressions of a regular expression and typically capture substrings of an input string. Grouping constructs include the language elements listed in the following table. For more information, see Grouping Constructs.

 

Grouping construct	Description	Pattern	Matches
( subexpression )	Captures the matched subexpression and assigns it a zero-based ordinal number.	(\w)\1	"ee" in "deep"
(?< name > subexpression)	Captures the matched subexpression into a named group.	(?<double>\w)\k<double>	"ee" in "deep"
(?< name1 - name2 > subexpression)	Defines a balancing group definition. For more information, see the "Balancing Group Definition" section in Grouping Constructs.	(((?'Open'\()[^\(\)]*)+((?'Close-Open'\))[^\(\)]*)+)*(?(Open)(?!))$	"((1-3)*(3-1))" in "3+2^((1-3)*(3-1))"
(?: subexpression)	Defines a noncapturing group.	Write(?:Line)?	"WriteLine" in "Console.WriteLine()"
(?imnsx-imnsx: subexpression)	Applies or disables the specified options within subexpression. For more information, see Regular Expression Options.	A\d{2}(?i:\w+)\b	"A12xl", "A12XL" in "A12xl A12XL a12xl"
(?= subexpression)	Zero-width positive lookahead assertion.	\w+(?=\.)	"is", "ran", and "out" in "He is. The dog ran. The sun is out."
(?! subexpression)	Zero-width negative lookahead assertion.	\b(?!un)\w+\b	"sure", "used" in "unsure sure unity used"
(?<= subexpression)	Zero-width positive lookbehind assertion.	(?<=19)\d{2}\b	"99", "50", "05" in "1851 1999 1950 1905 2003"
(?<! subexpression)	Zero-width negative lookbehind assertion.	(?<!19)\d{2}\b	"51", "03" in "1851 1999 1950 1905 2003"
(?> subexpression)	Nonbacktracking (or "greedy") subexpression.	[13579](?>A+B+)	"1ABB", "3ABB", and "5AB" in "1ABB 3ABBC 5AB 5AC"

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Quantifiers

A quantifier specifies how many instances of the previous element (which can be a character, a group, or a character class) must be present in the input string for a match to occur. Quantifiers include the language elements listed in the following table. For more information, see Quantifiers.

 

Quantifier	Description	Pattern	Matches
*	Matches the previous element zero or more times.	\d*\.\d	".0", "19.9", "219.9"
+	Matches the previous element one or more times.	"be+"	"bee" in "been", "be" in "bent"
?	Matches the previous element zero or one time.	"rai?n"	"ran", "rain"
{ n }	Matches the previous element exactly n times.	",\d{3}"	",043" in "1,043.6", ",876", ",543", and ",210" in "9,876,543,210"
{ n ,}	Matches the previous element at least n times.	"\d{2,}"	"166", "29", "1930"
{ n , m }	Matches the previous element at least n times, but no more than m times.	"\d{3,5}"	"166", "17668" "19302" in "193024"
*?	Matches the previous element zero or more times, but as few times as possible.	\d*?\.\d	".0", "19.9", "219.9"
+?	Matches the previous element one or more times, but as few times as possible.	"be+?"	"be" in "been", "be" in "bent"
??	Matches the previous element zero or one time, but as few times as possible.	"rai??n"	"ran", "rain"
{ n }?	Matches the preceding element exactly n times.	",\d{3}?"	",043" in "1,043.6", ",876", ",543", and ",210" in "9,876,543,210"
{ n ,}?	Matches the previous element at least n times, but as few times as possible.	"\d{2,}?"	"166", "29", "1930"
{ n , m }?	Matches the previous element between n and m times, but as few times as possible.	"\d{3,5}?"	"166", "17668" "193", "024" in "193024"

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Backreference Constructs

A backreference allows a previously matched subexpression to be identified subsequently in the same regular expression. The following table lists the backreference constructs supported by regular expressions in the .NET Framework. For more information, see Backreference Constructs.

 

Backreference construct	Description	Pattern	Matches
\ number	Backreference. Matches the value of a numbered subexpression.	(\w)\1	"ee" in "seek"
\k< name >	Named backreference. Matches the value of a named expression.	(?<char>\w)\k<char>	"ee" in "seek"

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Alternation Constructs

Alternation constructs modify a regular expression to enable either/or matching. These constructs include the language elements listed in the following table. For more information, see Alternation Constructs.

 

Alternation construct	Description	Pattern	Matches
|	Matches any one element separated by the vertical bar (|) character.	th(e|is|at)	"the", "this" in "this is the day. "
(?( expression ) yes | no )	Matches yes if expression matches; otherwise, matches the optional no part. expression is interpreted as a zero-width assertion.	(?(A)A\d{2}\b|\b\d{3}\b)	"A10", "910" in "A10 C103 910"
(?( name ) yes | no )	Matches yes if the named capture name has a match; otherwise, matches the optional no.	(?<quoted>")?(?(quoted).+?"|\S+\s)	Dogs.jpg, "Yiska playing.jpg" in "Dogs.jpg "Yiska playing.jpg""

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Substitutions

Substitutions are regular expression language elements that are supported in replacement patterns. For more information, see Substitutions. The metacharacters listed in the following table are atomic zero-width assertions.

 

Character	Description	Pattern	Replacement pattern	Input string	Result string
$ number	Substitutes the substring matched by group number.	\b(\w+)(\s)(\w+)\b	$3$2$1	"one two"	"two one"
${ name }	Substitutes the substring matched by the named group name.	\b(?<word1>\w+)(\s)(?<word2>\w+)\b	${word2} ${word1}	"one two"	"two one"
$$	Substitutes a literal "$".	\b(\d+)\s?USD	$$$1	"103 USD"	"$103"
$&	Substitutes a copy of the whole match.	(\$*(\d*(\.+\d+)?){1})	**$&	"$1.30"	"**$1.30**"
$`	Substitutes all the text of the input string before the match.	B+	$`	"AABBCC"	"AAAACC"
$'	Substitutes all the text of the input string after the match.	B+	$'	"AABBCC"	"AACCCC"
$+	Substitutes the last group that was captured.	B+(C+)	$+	"AABBCCDD"	AACCDD
$_	Substitutes the entire input string.	B+	$_	"AABBCC"	"AAAABBCCCC"

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Miscellaneous Constructs

Miscellaneous constructs either modify a regular expression pattern or provide information about it. The following table lists the miscellaneous constructs supported by the .NET Framework. For more information, see Miscellaneous Constructs.

 

Construct	Definition	Example
(?imnsx-imnsx)	Sets or disables options such as case insensitivity in the middle of a pattern. For more information, see Regular Expression Options.	\bA(?i)b\w+\b matches "ABA", "Able" in "ABA Able Act"
(?# comment)	Inline comment. The comment ends at the first closing parenthesis.	\bA(?#Matches words starting with A)\w+\b
# [to end of line]	X-mode comment. The comment starts at an unescaped # and continues to the end of the line.	(?x)\bA\w+\b#Matches words starting with A

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See Also

Reference

System.Text.RegularExpressions

Regex

Concepts

Regular Expression Classes

Other Resources

.NET Framework Regular Expressions

Best Practices for Regular Expressions in the .NET Framework

Regular Expression Examples