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Engine: Boost.Regex | regexp-features

Engine: Boost.Regex

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Languages

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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.
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).

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

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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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A Posix Character Class is a member of a Character Class set that specifies a named, pre-defined set of characters.

Syntax

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[[:name:]] — Where name is in a set of predefined names. Matches any character in the set.

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Feature: Negated Posix Character Classes

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Feature: Collating Elements

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A Collating Element is one or more characters that collate as a single unit.

Syntax

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[[.col.]] — Matches the collating element col if col is treated as a single collating unit in the current locale. Example: [[.ae.]-c] matches a single character in the range “ae”-“c”. col may also be specified by a symbolic name.

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Feature: Equivalence Classes

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An Equivalence Class matches any character or collating element with the same primary sort key as the provided collating element.

Syntax

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[[=col=]] — Matches any character with the same primary sort key as col. Example: [[=a=]] matches a, À, Á, Â, Ã, Ä, Å, A, à, á, â, etc.

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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 — A decimal digit character in the range 0-9. Equivalent to [0-9].
\D — Any character not in the range 0-9. Equivalent to [^0-9].
\w — Any “word” character. Equivalent to [a-zA-Z0-9_].
\W — Any non-“word” character. Equivalent to [^a-zA-Z0-9_].
\s — Any whitespace character.
\S — Any non-whitespace character.
\l — Any lower-case character.
\L — Any non-lower-case character.
\u — Any upper-case character.
\U — Any non-upper-case character.
\h — Any horizontal whitespace character.
\H — Any non-horizontal whitespace character.
\v — Any vertical whitespace character.
\V — Any non-vertical whitespace character.

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Feature: Line Endings Escape

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A Line Endings Escape is an Atom that matches any line ending character sequence.

Syntax

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\R — Equivalent to (?>\r\n?|[\x0A-\x0C\x85\u{2028}\u{2029}])

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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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\pX — Where X is a single character. Matches a character that has the property X.
\p{name} — Where name is a predefined property name. Matches a character that has the property name.
\PX — Where X is a single character. Matches a character that does not have the property X.
\P{name} — Where name is a predefined property name. Matches a character that does not have the 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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Feature: Character Class Symmetric Difference

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Feature: Character Class Complement

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Feature: Quoted Characters

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Quoted Characters are a sequence of characters treated as literal characters rather than RegExp characters.

Syntax

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\Q…\E — All characters following \Q and preceding the next \E are treated as literal characters. Example: \Q.+\E matches .+ but not aa.
\Q… — If there is no trailing \E, all characters until the end of the pattern are treated as literal characters.

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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Possessive Quantifiers are like greedy (i.e., regular) quantifiers, except that backtracking is not performed.

Syntax

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*+ — Match zero or more characters without backtracking.
++ — Match one or more characters without backtracking.
?+ — Match zero or one characters without backtracking.
{n,}+ — Where n is an integer. Matches the preceding Atom at-least n times without backtracking.
{n,m}+ — Where n and m are integers, and m >= n. Matches the preceding Atom at-least n times and at-most m times without backtracking.

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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 in the range 1-9. Matches the same string as the capture group n.
\gn — Where n is in the range 1-9. Matches the same string as the capture group n.
\g-n — Where n is in the range 1-9. Matches the nth previous capture group.
\g{n} — Where n is an integer >= 0. Matches the same string as the capture group n.
\g{-n} — Where n is an integer >= 0. Matches the nth previous capture group.
\g{name} — Matches the named capture group with the name name.
\k<name> — Matches 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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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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(?imsx-imsx) - 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.
(?imsx-imsx:…) - 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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A Branch Reset resets the subexpression count at the start of each Alternative (|), which affects numbering for Backreferences and captured results returned from the matching algorithm.

Syntax

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(?|…) — Resets the subexpression count at the start of each Alternative.

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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A Recursive Expression provides a mechanism for re-evaluating a capture group inside of itself, to handle cases such as matching balanced parenthesis or brackets, etc.

Syntax

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(?R) — Reevaluates the entire pattern starting at the current position.
(?0) — Reevaluates the entire pattern starting at the current position.
(?n) — Where n is an integer >= 1. Re-evaluates the capture group whose offset is n.
(?-n) — Where n is an integer >= 1. Re-evaluates the capture group whose offset is the nth capture group declared to the left of the current Atom. Example: (?-1) would revaluate the last declared capture group.
(?+n) — Where n is an integer >= 1. Re-evaluates the capture group whose offset is the nth capture group declared to the right of the current Atom. Example: (?+1) would evaluate the next declared capture group.
(?&name) — Re-evaluates the named capture group with the provided name.

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

Conditions

The following conditions are supported:

(?(?=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.
(?(n)…) — Evaluates to true if the capture group at offset n was successfully matched; Otherwise, evaluates to false.
(?(<name>)…) — Evaluates to true if the named capture group with the name name was successfully matched; Otherwise, evaluates to false.
(?('name')…) — Evaluates to true if the named capture group with the name name was successfully matched; Otherwise, evaluates to false.
(?(R)…) — Evaluates to true if inside a recursive expression; Otherwise, evaluates to false.
(?(Rn)…) — Evaluates to true if inside a recursive expression for the capture group at offset n; Otherwise, evaluates to false.
(?(R&name)…) — Evaluates to true if inside a recursive expression for the named capture group with the name name; Otherwise, evaluates to false.
(?(DEFINE)…) — Always evaluates to false. This allows you to define Subroutines.

Feature: Subroutines

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A Subroutine is a pre-defined capture group or named capture group that can be reused in multiple places within the pattern to re-evaluate the subexpression from the group.

Syntax

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(?(DEFINE)…) — Defines a set of reusable capture groups that can be referenced elsewhere in the pattern.
(?n) — Where n is an integer >= 1. Evaluates the capture group whose offset is n.
(?-n) — Where n is an integer >= 1. Evaluates the capture group whose offset is the nth capture group declared to the left of the current Atom. Example: (?-1) would revaluate the last declared capture group.
(?+n) — Where n is an integer >= 1. Evaluates the capture group whose offset is the nth capture group declared to the right of the current Atom. Example: (?+1) would evaluate the next declared capture group.
(?&name) — Evaluates the named capture group with the provided name.

Example

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(?(DEFINE)
  (?<Year>\d{4}|[+-]\d{5,})
  (?<Month>0[1-9]|1[0-2])
  (?<Day>0[1-9]|2[0-9]|3[01])
)
(?<Date>(?&Year)-(?&Month)-(?&Day)|(?&Year)(?&Month)(?&Day))

Feature: Callouts

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Feature: Backtracking Control Verbs

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