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Engine: .NET | regexp-features

Engine: .NET

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Features

The following features are supported:

The following features are not supported:

Feature: Flags

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Flags control certain aspects of the matching behavior of a pattern.

Syntax

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The following flags are supported:

i — Ignore Case. Matches character classes using a case-insensitive comparison.
m — Multiline. Causes the anchors ^ and $ to match the start and end of each line (respectively), rather than the start and end of the input.
n — Explicit captures. Regular Capturing Groups are not captured, only Named Capturing Groups are captured.
s — Singleline. Causes the wildcard . to match newline characters.
x — Extended Mode. Ignores whitespace in a pattern. Spaces must instead be represented by \s or \ (an escaped space). Allows x-mode comments.

See Also

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Modifiers

Feature: Anchors

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Anchors match the start or end of a line.

Syntax

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^ — Matches the start of a line when the m (multiline) flag is set. Otherwise, matches the start of the input.
$ — Matches the end of a line when the m (multiline) flag is set. Otherwise, matches the end of the input.

See Also

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Feature: Buffer Boundaries

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A Buffer Boundary is an Atom that matches the start or the end of the input. This differs slightly from ^ and $ which can be affected by RegExp flags like m.

Syntax

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\A — Matches the start of the input.
\z — Matches the end of the input.
\Z — A zero-width assertion consisting of an optional newline at the end of the buffer. Equivalent to (?=\n?\z).

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Feature: Word Boundaries

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A Word Boundary is an Atom that matches the start or the end of a word.

Syntax

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\b — Matches the start or the end of a word.
\B — Matches when not at the start or the end of a word.

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Feature: Text Segment Boundaries

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Feature: Continuation Escape

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A Continuation Escape is a zero-width assertion that matches either the start of the input or the start of the last match.

Syntax

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\G — Matches either the start of the input or the start of the last match.

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Feature: Alternatives

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An Alternative represents two or more branches in a pattern. If first branch of a pattern fails to match, each alternative is attempted from left to right until a match is found.

Syntax

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…|… — Matches the pattern to the left of the |. If that fails, matches the pattern to the right of |.

Feature: Wildcard

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A Wildcard matches a single, non-newline character.

Syntax

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. — Matches any character except newline characters. If the s (single-line) flag is set then this matches any character.

Feature: Character Classes

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A Character Class is an Atom that specifies a set of characters to match a single character in the set.

Syntax

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[…] — Where … is one or more single characters or character class escapes, excluding ^ at the start and - between two entries in the set. Matches a character in the set. Example: [abc] matches a, b, or c.
[^…] — Where … is one or more single characters or character class escapes, excluding - between two entries in the set. Matches any character not in the set. Example: [^abc] matches d, e, or f, etc., but not a, b, or c.
[a-z] — Where a and z are single characters or character escapes. Matches any character in the range between a and z (inclusive). Example: [a-c] matches a, b, or c, but not d.

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Feature: Posix Character Classes

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Feature: Negated Posix Character Classes

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Feature: Collating Elements

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Feature: Equivalence Classes

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Feature: Character Class Escapes

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A Character Class Escape is a single character escape that represents an entire character class. They can be used as an element of a Character Class or as an Atom. It is often the case that a lower-case escape character is the inclusive set, while an upper-case variant of the same character excludes that set.

Syntax

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\d — Any digit character. Equivalent to \p{Nd} unless in ECMAScript-compliant mode, in which case \d is equivalent to [0-9].
\D — Any non-digit character. Equivalent to \P{Nd} unless in ECMAScript-compliant mode, in which case \D is equivalent to [^0-9].
\w — Any “word” character. Equivalent to [\p{Ll}\p{Lu}\p{Lt}\p{Lo}\p{Lm}\p{Mn}\p{Nd}\p{Pc}] unless in ECMAScript-compliant mode, in which case \w is equivalent to [a-zA-Z0-9_].
\W — Any non-“word” character. Equivalent to [^\p{Ll}\p{Lu}\p{Lt}\p{Lo}\p{Lm}\p{Mn}\p{Nd}\p{Pc}] unless in ECMAScript-compliant mode, in which case \W is equivalent to [^a-zA-Z0-9_].
\s — Any whitespace character. Equivalent to [\f\n\r\t\v\x85\p{Z}] unless in ECMAScript-compliant mode, in which case \s is equivalent to [ \f\n\r\t\v].
\S — Any non-whitespace character. Equivalent to [^\f\n\r\t\v\x85\p{Z}] unless in ECMAScript-compliant mode, in which case \s is equivalent to [^ \f\n\r\t\v].

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Feature: Line Endings Escape

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Feature: Character Property Escapes

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A Character Property Escape is an escape sequence used to match a character with a specific character property.

Syntax

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\p{name} — Where name is a predefined unicode property name. Matches a character that has the unicode property name.
\P{name} — Where name is a predefined unicode property name. Matches a character that does not have the unicode property name.

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Feature: Character Class Nested Set

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Feature: Character Class Intersection

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Feature: Character Class Union

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Feature: Character Class Subtraction

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Character Class Subtraction allows you to exclude a class of characters from another class of characters in a character class.

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Feature: Character Class Symmetric Difference

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Feature: Character Class Complement

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Feature: Quoted Characters

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Feature: Quantifiers

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Quantifiers specify repetition of an Atom. By default, quantifiers are “greedy” in that they attempt to match as many instances of the preceding Atom as possible to satisfy the pattern before backtracking.

Syntax

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* — Matches the preceding Atom zero or more times. Example: a*b matches b, ab, aab, aaab, etc.
+ — Matches the preceding Atom one or more times. Example: a+b matches ab, aab, aaab, etc., but not b.
? — Matches the preceding Atom zero or one times. Example: a?b matches b, ab.
{n} — Where n is an integer. Matches the preceding Atom exactly n times. Example: a{2} matches aa but not a or aaa.
{n,} — Where n is an integer. Matches the preceding Atom at-least n times. Example: a{2,} matches aa, aaa, aaaa, etc., but not a.
{n,m} — Where n and m are integers, and m >= n. Matches the preceding Atom at-least n times and at-most m times. Example: a{2,3} matches aa, aaa, aaaa, etc., but not a or aaaa.

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Feature: Lazy Quantifiers

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Lazy Quantifiers specify repetition of an Atom, but attempt to match as few instances of the preceding Atom as possible to satisfy the pattern before advancing.

Syntax

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*? — Matches the preceding Atom zero or more times.
+? — Matches the preceding Atom one or more times.
?? — Matches the preceding Atom zero or one times.
{n}? — Where n is an integer. Matches the preceding Atom exactly n times.
{n,}? — Where n is an integer. Matches the preceding Atom at-least n times.
{n,m}? — Where n and m are integers, and m >= n. Matches the preceding Atom at-least n times and at-most m times.

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Feature: Possessive Quantifiers

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Feature: Capturing Groups

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A Capturing Group is a subexpression that can be treated as an Atom and can be repeated using Quantifiers and referenced using Backreferences by index. A Capturing Group can be captured and returned by the matching algorithm.

Syntax

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(…) — Groups the subexpression as a single Atom. The result is captured and returned by the matching algorithm.

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Feature: Named Capturing Groups

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A Named Capturing Group is a subexpression that can be captured and returned by the matching algorithm. A Named Capturing Group is also an Atom and can be repeated using Quantifiers and referenced using Backreferences by name.

Syntax

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(?<name>…) — Groups the subexpression as a single Atom associated with the provided name. The result is captured and returned by the matching algorithm.
(?'name'…) — Groups the subexpression as a single Atom associated with the provided name. The result is captured and returned by the matching algorithm.

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Feature: Non-Capturing Groups

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A Non-capturing Group is a subexpression that can be treated as an Atom and can be repeated using Quantifiers but cannot be referenced using Backreferences. A Non-capturing Group is not captured by the matching algorithm.

Syntax

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(?:…) — Groups the subexpression as a single Atom.

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Feature: Backreferences

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Backreferences allow a pattern to re-match a previously matched capture group^{1 2} either by number (n) or by name.

Syntax

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\n — Where n is a decimal digit in the range 1-9. Matches the same string as the capture group n.
\k<name> — Matches the same string as the named capture group with the name name.

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A Comment is a sequence of characters that is ignored by pattern matching and can be used to document a pattern.

Syntax

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(?#…) — The entire expression is removed from the pattern. A comment may not contain other ( or ) characters.

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Line Comments

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A Line Comment is a sequence of characters starting with # and ending with \n (or the end of the pattern) that is ignored by pattern matching and can be used to document a pattern.

Syntax

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#…\n — The rest of the line starting from # is removed from the pattern. Only supported when the x (extended mode) RegExp flag is set.

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Comments

Feature: Modifiers

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Modifiers allow you to change the currently active RegExp flags within a subexpression.

Syntax

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(?imnsx-imnsx) - Sets or unsets (using -) the specified RegExp flags starting at the current position until the next closing ) or the end of the pattern. Example: (?-i)A(?i)B(?-i)C matches ABC, AbC.
(?imnsx-imnsx:…) - Sets or unsets (using -) the specified RegExp flags for the subexpression. Example: (?-i:A(?i:B)C) matches ABC, AbC.

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Flags

Feature: Branch Reset

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Feature: Lookahead

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A Lookahead is a zero-width assertion that matches if the provided pattern would match the characters to the right of the current position.

Syntax

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(?=…) — Positive Lookahead. Matches if the provided pattern would match but does not advance the current position.
(?!…) — Negative Lookahead. Matches if the provided pattern would not match, but does not advance the current position.

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Lookbehind

Feature: Lookbehind

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A Lookbehind is a zero-width assertion that matches if the provided pattern would match the characters to the left of the current position.

Syntax

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(?<=…) — Positive Lookbehind. Matches if the provided pattern would match the preceding characters, but does not advance the current position. The pattern must have a fixed length (unbounded quantifiers are not permitted).
(?<!…) — Negative Lookbehind. Matches if the provided pattern would not match the preceding characters, but does not advance the current position. The pattern must have a fixed length (unbounded quantifiers are not permitted).

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Lookahead

Feature: Non-Backtracking Expressions

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A Non-Backtracking Expression is matched independent of neighboring patterns, and will not backtrack in the event of a failed match. This is often used to improve performance.

Syntax

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(?>…) — Matches the provided pattern, but no backtracking is performed if the match fails.

Feature: Recursion

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.NET’s Regex class supports limited recursion through balancing groups. A Balancing Group uses the name of a previously defined named capture group as a stack, storing the interval between the current position and the last instance of the named capture group in the current name. You can then use a conditional expression to fail the matching algorithm if the previously defined named capture group’s stack is not empty when the match completes.

Syntax

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(?<name1-name1>…) — Where name1 as the current group name and name2 is a previously defined group. Stores the interval between name2 and the current position in name1 and deletes name2. Deleting name2 reveals the previous definition of name2 allowing it to be used as a stack for captures.
(?<-name2>…) — Where name2 is a previously defined group. Stores the interval between name2 and the current position as the current capture and deletes name2. Deleting name2 reveals the previous definition of name2 allowing it to be used as a stack for captures.
(?'name1-name1'…) — Where name1 as the current group name and name2 is a previously defined group. Stores the interval between name2 and the current position in name1 and deletes name2. Deleting name2 reveals the previous definition of name2 allowing it to be used as a stack for captures.
(?'-name2'…) — Where name2 is a previously defined group. Stores the interval between name2 and the current position as the current capture and deletes name2. Deleting name2 reveals the previous definition of name2 allowing it to be used as a stack for captures.

Example

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The following shows an example of matching balanced < and > brackets (assuming x mode for a multiline regular expression):

^                               # matches the beginning of the input

[^<>]*                          # matches any number of non <> characters
(
    ((?'Open'<)[^<>]*)+         # matches an opening < followed by any number of non <> characters

    ((?'Close-Open'>)[^<>]*)+   # matches a closing > followed by any number of non <> characters,
                                # deleting the last match for 'Open'

)*                              # matches the preceding expression zero or more times.

(?(Open)(?!))                   # condition that fails the match if there are any remaining 'Open' matches

$                               # matches the end of the input

On a single line, the expression looks like this:

^[^<>]*(((?'Open'<)[^<>]*)+((?'Close-Open'>)[^<>]*)+)*(?(Open)(?!))$

Feature: Conditional Expressions

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A Conditional Expression checks a condition and evaluates its first alternative if the condition is true; otherwise, it evaluates its second alternative.

Syntax

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(?(condition)condition|condition) — Matches yes-pattern if condition is true; otherwise, matches no-pattern.
(?(condition)condition) — Matches yes-pattern if condition is true; otherwise, matches the empty string.

Conditions

The following conditions are supported:

(?(n)…) — Evaluates to true if the capture group at offset n was successfully matched; Otherwise, evaluates to false. If n does not correspond to a capture group, an error is thrown.
(?(name)…) — Evaluates to true if the named capture group with the name name was successfully matched; Otherwise, evaluates to false. If name does not correspond to a named capture group, name is interpeted as a lookahead pattern.
(?(test-pattern)…) — Evaluates to test-patterntruetest-pattern if a lookahead for test-pattern matches; Otherwise, evaluates to test-patternfalsetest-pattern. Equivalent to: (?(?=test-pattern)…).
(?(?=test-pattern)…) — Evaluates to true if a lookahead for test-pattern matches; Otherwise, evaluates to false.
(?(?!test-pattern)…) — Evaluates to true if a negative lookahead for test-pattern matches; Otherwise, evaluates to false.
(?(?<=test-pattern)…) — Evaluates to true if a lookbehind for test-pattern matches; Otherwise, evaluates to false.
(?(?<!test-pattern)…) — Evaluates to true if a negative lookbehind for test-pattern matches; Otherwise, evaluates to false.

Feature: Subroutines

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Feature: Callouts

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Feature: Backtracking Control Verbs

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