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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 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.
We've also provided this information in two formats that you can download and print for easy reference:
Download in Word (.docx) format
Download in PDF (.pdf) format
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
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."
Matches a vertical tab, \u000B.
[\v]{2,}
"\v\v\v"
in
"\v\v\v"
Matches a form feed, \u000C.
[\f]{2,}
"\f\f\f"
in
"\f\f\f"
Matches a new line, \u000A.
\r\n(\w+)
"\r\nThese"
in
"\r\nThese are\ntwo lines."
Matches an escape, \u001B.
"\x001B"
in
"\x001B"
\
nnn
Uses octal representation to specify a character (
nnn
consists of two or 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.
"\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
, and
\.
is the same as
\x2E
. This allows the regular expression engine to disambiguate language elements (such as * or ?) and character literals (represented by
\*
or
\?
).
\d+[\+-x\*]\d+
"2+2"
and
"3*9"
in
"(2+2) * 3*9"
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.
"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
.
To match a literal period character (. or
\u002E
), you must precede it with the escape character (
\.
).
"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"
Matches any
word character
.
"I"
,
"D"
,
"A"
,
"1"
,
"3"
in
"ID A1.3"
Matches any
non-word character
.
" "
,
"."
in
"ID A1.3"
Matches any
white-space character
.
"D "
in
"ID A1.3"
Matches any
non-white-space character
.
" _"
in
"int __ctr"
Matches any
decimal digit
.
"4"
in
"4 = IV"
Matches any
character other than a decimal digit
.
" "
,
"="
,
" "
,
"I"
,
"V"
in
"4 = IV"
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
.
Assertion
Description
Pattern
Matches
By default, the match must start at the beginning of the string; in multiline mode, it must start at the beginning of the line.
^\d{3}
"901"
in
"901-333-"
By default, the match must occur at the end of the string or before
\n
at the end of the string; in multiline mode, it must occur before the end of the line or before
\n
at the end of the line.
-\d{3}$
"-333"
in
"-901-333"
The match must occur at the start of the string.
\A\d{3}
"901"
in
"901-333-"
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"
The match must occur at the end of the string.
-\d{3}\z
"-333"
in
"-901-333"
The match must occur at the point where the previous match ended, or if there was no previous match, at the position in the string where matching started.
\G\(\d\)
"(1)"
,
"(3)"
,
"(5)"
in
"(1)(3)(5)[7](9)"
The match must occur on a boundary between a
\w
(alphanumeric) and a
\W
(nonalphanumeric) character.
\b\w+\s\w+\b
"them theme"
,
"them them"
in
"them theme them them"
The match must not occur on a
\b
boundary.
\Bend\w*\b
"ends"
,
"ender"
in
"end sends endure lender"
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 one-based ordinal number.
(\w)\1
"ee"
in
"deep"
(?<
name
>
subexpression
)
or
(?'
name
'
subexpression
)
Captures the matched subexpression into a named group.
(?<double>\w)\k<double>
"ee"
in
"deep"
(?<
name1
-
name2
>
subexpression
)
or
(?'
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()"
"Write"
in
"Console.Write(value)"
(?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.
\b\w+\b(?=.+and.+)
"cats"
,
"dogs"
in
"cats, dogs and some mice."
(?!
subexpression
)
Zero-width negative lookahead assertion.
\b\w+\b(?!.+and.+)
"and"
,
"some"
,
"mice"
in
"cats, dogs and some mice."
(?<=
subexpression
)
Zero-width positive lookbehind assertion.
\b\w+\b(?<=.+and.+)
———————————
\b\w+\b(?<=.+and.*)
"some"
,
"mice"
in
"cats, dogs and some mice."
————————————
"and"
,
"some"
,
"mice"
in
"cats, dogs and some mice."
(?<!
subexpression
)
Zero-width negative lookbehind assertion.
\b\w+\b(?<!.+and.+)
———————————
\b\w+\b(?<!.+and.*)
"cats"
,
"dogs"
,
"and"
in
"cats, dogs and some mice."
————————————
"cats"
,
"dogs"
in
"cats, dogs and some mice."
(?>
subexpression
)
Atomic group.
(?>a|ab)c
"ac"
in
"ac"
nothing
in
"abc"
Lookarounds at a glance
When the regular expression engine hits a
lookaround expression
, it takes a substring reaching from the current position to the start (lookbehind) or end (lookahead) of the original string, and then runs
Regex.IsMatch
on that substring using the lookaround pattern. Success of this subexpression's result is then determined by whether it's a positive or negative assertion.
Lookaround
Function
(?=check)
Positive Lookahead
Asserts that what immediately follows the current position in the string is "check"
(?<=check)
Positive Lookbehind
Asserts that what immediately precedes the current position in the string is "check"
(?!check)
Negative Lookahead
Asserts that what immediately follows the current position in the string is not "check"
(?<!check)
Negative Lookbehind
Asserts that what immediately precedes the current position in the string is not "check"
Once they have matched,
atomic groups
won't be re-evaluated again, even when the remainder of the pattern fails due to the match. This can significantly improve performance when quantifiers occur within the atomic group or the remainder of the pattern.
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 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.
a.*?c
"abc"
in
"abcbc"
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??"
"ra"
in
"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"
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 .NET. For more information, see
Backreference Constructs
.
Backreference 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
)
or
(?(
expression
)
yes
)
Matches
yes
if the regular expression pattern designated by
expression
matches; otherwise, matches the optional
no
part.
expression
is interpreted as a zero-width assertion.
To avoid ambiguity with a named or numbered capturing group, you can optionally use an explicit assertion, like this:
(?( (?=
expression
) )
yes
|
no
)
(?(A)A\d{2}\b|\b\d{3}\b)
"A10"
,
"910"
in
"A10 C103 910"
(?(
name
)
yes
|
no
)
or
(?(
name
)
yes
)
Matches
yes
if
name
, a named or numbered capturing group, has a match; otherwise, matches the optional
no
.
(?<quoted>")?(?(quoted).+?"|\S+\s)
"Dogs.jpg "
,
"\"Yiska playing.jpg\""
in
"Dogs.jpg \"Yiska playing.jpg\""
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
${
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
"103 USD"
"$103"
Substitutes a copy of the whole match.
\$?\d*\.?\d+
**$&**
"$1.30"
"**$1.30**"
Substitutes all the text of the input string before the match.
"AABBCC"
"AAAACC"
Substitutes all the text of the input string after the match.
"AABBCC"
"AACCCC"
Substitutes the last group that was captured.
B+(C+)
"AABBCCDD"
"AACCDD"
Substitutes the entire input string.
"AABBCC"
"AAAABBCCCC"
Regular Expression Options
You can specify options that control how the regular expression engine interprets a regular expression pattern. Many of these options can be specified either inline (in the regular expression pattern) or as one or more
RegexOptions
constants. This quick reference lists only inline options. For more information about inline and
RegexOptions
options, see the article
Regular Expression Options
.
You can specify an inline option in two ways:
By using the
miscellaneous construct
(?imnsx-imnsx)
, where a minus sign (-) before an option or set of options turns those options off. For example,
(?i-mn)
turns case-insensitive matching (
i
) on, turns multiline mode (
m
) off, and turns unnamed group captures (
n
) off. The option applies to the regular expression pattern from the point at which the option is defined, and is effective either to the end of the pattern or to the point where another construct reverses the option.
By using the
grouping construct
(?imnsx-imnsx:
subexpression
)
, which defines options for the specified group only.
The .NET regular expression engine supports the following inline options:
Option
Description
Pattern
Matches
Use case-insensitive matching.
\b(?i)a(?-i)a\w+\b
"aardvark"
,
"aaaAuto"
in
"aardvark AAAuto aaaAuto Adam breakfast"
Use multiline mode.
^
and
$
match the beginning and end of a line, instead of the beginning and end of a string.
For an example, see the "Multiline Mode" section in
Regular Expression Options
.
Do not capture unnamed groups.
For an example, see the "Explicit Captures Only" section in
Regular Expression Options
.
Use single-line mode.
For an example, see the "Single-line Mode" section in
Regular Expression Options
.
Ignore unescaped white space in the regular expression pattern.
\b(?x) \d+ \s \w+
"1 aardvark"
,
"2 cats"
in
"1 aardvark 2 cats IV centurions"
Miscellaneous Constructs
Miscellaneous constructs either modify a regular expression pattern or provide information about it. The following table lists the miscellaneous constructs supported by .NET. 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
See also
System.Text.RegularExpressions
System.Text.RegularExpressions.Regex
Regular Expressions
Regular Expression Classes
Regular Expressions - Quick Reference (download in Word format)
Regular Expressions - Quick Reference (download in PDF format)
