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260 lines
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260 lines
10 KiB
# Unicode conformance
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This document describes the regex crate's conformance to Unicode's
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[UTS#18](https://unicode.org/reports/tr18/)
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report, which lays out 3 levels of support: Basic, Extended and Tailored.
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Full support for Level 1 ("Basic Unicode Support") is provided with two
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exceptions:
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1. Line boundaries are not Unicode aware. Namely, only the `\n`
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(`END OF LINE`) character is recognized as a line boundary.
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2. The compatibility properties specified by
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[RL1.2a](https://unicode.org/reports/tr18/#RL1.2a)
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are ASCII-only definitions.
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Little to no support is provided for either Level 2 or Level 3. For the most
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part, this is because the features are either complex/hard to implement, or at
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the very least, very difficult to implement without sacrificing performance.
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For example, tackling canonical equivalence such that matching worked as one
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would expect regardless of normalization form would be a significant
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undertaking. This is at least partially a result of the fact that this regex
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engine is based on finite automata, which admits less flexibility normally
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associated with backtracking implementations.
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## RL1.1 Hex Notation
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[UTS#18 RL1.1](https://unicode.org/reports/tr18/#Hex_notation)
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Hex Notation refers to the ability to specify a Unicode code point in a regular
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expression via its hexadecimal code point representation. This is useful in
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environments that have poor Unicode font rendering or if you need to express a
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code point that is not normally displayable. All forms of hexadecimal notation
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are supported
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\x7F hex character code (exactly two digits)
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\x{10FFFF} any hex character code corresponding to a Unicode code point
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\u007F hex character code (exactly four digits)
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\u{7F} any hex character code corresponding to a Unicode code point
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\U0000007F hex character code (exactly eight digits)
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\U{7F} any hex character code corresponding to a Unicode code point
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Briefly, the `\x{...}`, `\u{...}` and `\U{...}` are all exactly equivalent ways
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of expressing hexadecimal code points. Any number of digits can be written
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within the brackets. In contrast, `\xNN`, `\uNNNN`, `\UNNNNNNNN` are all
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fixed-width variants of the same idea.
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Note that when Unicode mode is disabled, any non-ASCII Unicode codepoint is
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banned. Additionally, the `\xNN` syntax represents arbitrary bytes when Unicode
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mode is disabled. That is, the regex `\xFF` matches the Unicode codepoint
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U+00FF (encoded as `\xC3\xBF` in UTF-8) while the regex `(?-u)\xFF` matches
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the literal byte `\xFF`.
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## RL1.2 Properties
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[UTS#18 RL1.2](https://unicode.org/reports/tr18/#Categories)
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Full support for Unicode property syntax is provided. Unicode properties
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provide a convenient way to construct character classes of groups of code
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points specified by Unicode. The regex crate does not provide exhaustive
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support, but covers a useful subset. In particular:
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* [General categories](https://unicode.org/reports/tr18/#General_Category_Property)
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* [Scripts and Script Extensions](https://unicode.org/reports/tr18/#Script_Property)
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* [Age](https://unicode.org/reports/tr18/#Age)
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* A smattering of boolean properties, including all of those specified by
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[RL1.2](https://unicode.org/reports/tr18/#RL1.2) explicitly.
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In all cases, property name and value abbreviations are supported, and all
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names/values are matched loosely without regard for case, whitespace or
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underscores. Property name aliases can be found in Unicode's
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[`PropertyAliases.txt`](https://www.unicode.org/Public/UCD/latest/ucd/PropertyAliases.txt)
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file, while property value aliases can be found in Unicode's
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[`PropertyValueAliases.txt`](https://www.unicode.org/Public/UCD/latest/ucd/PropertyValueAliases.txt)
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file.
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The syntax supported is also consistent with the UTS#18 recommendation:
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* `\p{Greek}` selects the `Greek` script. Equivalent expressions follow:
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`\p{sc:Greek}`, `\p{Script:Greek}`, `\p{Sc=Greek}`, `\p{script=Greek}`,
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`\P{sc!=Greek}`. Similarly for `General_Category` (or `gc` for short) and
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`Script_Extensions` (or `scx` for short).
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* `\p{age:3.2}` selects all code points in Unicode 3.2.
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* `\p{Alphabetic}` selects the "alphabetic" property and can be abbreviated
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via `\p{alpha}` (for example).
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* Single letter variants for properties with single letter abbreviations.
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For example, `\p{Letter}` can be equivalently written as `\pL`.
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The following is a list of all properties supported by the regex crate (starred
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properties correspond to properties required by RL1.2):
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* `General_Category` \* (including `Any`, `ASCII` and `Assigned`)
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* `Script` \*
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* `Script_Extensions` \*
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* `Age`
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* `ASCII_Hex_Digit`
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* `Alphabetic` \*
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* `Bidi_Control`
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* `Case_Ignorable`
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* `Cased`
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* `Changes_When_Casefolded`
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* `Changes_When_Casemapped`
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* `Changes_When_Lowercased`
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* `Changes_When_Titlecased`
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* `Changes_When_Uppercased`
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* `Dash`
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* `Default_Ignorable_Code_Point` \*
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* `Deprecated`
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* `Diacritic`
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* `Emoji`
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* `Emoji_Presentation`
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* `Emoji_Modifier`
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* `Emoji_Modifier_Base`
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* `Emoji_Component`
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* `Extended_Pictographic`
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* `Extender`
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* `Grapheme_Base`
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* `Grapheme_Cluster_Break`
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* `Grapheme_Extend`
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* `Hex_Digit`
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* `IDS_Binary_Operator`
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* `IDS_Trinary_Operator`
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* `ID_Continue`
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* `ID_Start`
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* `Join_Control`
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* `Logical_Order_Exception`
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* `Lowercase` \*
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* `Math`
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* `Noncharacter_Code_Point` \*
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* `Pattern_Syntax`
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* `Pattern_White_Space`
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* `Prepended_Concatenation_Mark`
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* `Quotation_Mark`
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* `Radical`
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* `Regional_Indicator`
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* `Sentence_Break`
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* `Sentence_Terminal`
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* `Soft_Dotted`
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* `Terminal_Punctuation`
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* `Unified_Ideograph`
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* `Uppercase` \*
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* `Variation_Selector`
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* `White_Space` \*
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* `Word_Break`
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* `XID_Continue`
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* `XID_Start`
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## RL1.2a Compatibility Properties
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[UTS#18 RL1.2a](https://unicode.org/reports/tr18/#RL1.2a)
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The regex crate only provides ASCII definitions of the
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[compatibility properties documented in UTS#18 Annex C](https://unicode.org/reports/tr18/#Compatibility_Properties)
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(sans the `\X` class, for matching grapheme clusters, which isn't provided
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at all). This is because it seems to be consistent with most other regular
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expression engines, and in particular, because these are often referred to as
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"ASCII" or "POSIX" character classes.
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Note that the `\w`, `\s` and `\d` character classes **are** Unicode aware.
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Their traditional ASCII definition can be used by disabling Unicode. That is,
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`[[:word:]]` and `(?-u)\w` are equivalent.
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## RL1.3 Subtraction and Intersection
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[UTS#18 RL1.3](https://unicode.org/reports/tr18/#Subtraction_and_Intersection)
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The regex crate provides full support for nested character classes, along with
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union, intersection (`&&`), difference (`--`) and symmetric difference (`~~`)
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operations on arbitrary character classes.
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For example, to match all non-ASCII letters, you could use either
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`[\p{Letter}--\p{Ascii}]` (difference) or `[\p{Letter}&&[^\p{Ascii}]]`
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(intersecting the negation).
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## RL1.4 Simple Word Boundaries
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[UTS#18 RL1.4](https://unicode.org/reports/tr18/#Simple_Word_Boundaries)
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The regex crate provides basic Unicode aware word boundary assertions. A word
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boundary assertion can be written as `\b`, or `\B` as its negation. A word
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boundary negation corresponds to a zero-width match, where its adjacent
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characters correspond to word and non-word, or non-word and word characters.
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Conformance in this case chooses to define word character in the same way that
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the `\w` character class is defined: a code point that is a member of one of
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the following classes:
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* `\p{Alphabetic}`
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* `\p{Join_Control}`
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* `\p{gc:Mark}`
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* `\p{gc:Decimal_Number}`
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* `\p{gc:Connector_Punctuation}`
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In particular, this differs slightly from the
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[prescription given in RL1.4](https://unicode.org/reports/tr18/#Simple_Word_Boundaries)
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but is permissible according to
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[UTS#18 Annex C](https://unicode.org/reports/tr18/#Compatibility_Properties).
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Namely, it is convenient and simpler to have `\w` and `\b` be in sync with
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one another.
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Finally, Unicode word boundaries can be disabled, which will cause ASCII word
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boundaries to be used instead. That is, `\b` is a Unicode word boundary while
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`(?-u)\b` is an ASCII-only word boundary. This can occasionally be beneficial
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if performance is important, since the implementation of Unicode word
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boundaries is currently sub-optimal on non-ASCII text.
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## RL1.5 Simple Loose Matches
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[UTS#18 RL1.5](https://unicode.org/reports/tr18/#Simple_Loose_Matches)
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The regex crate provides full support for case insensitive matching in
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accordance with RL1.5. That is, it uses the "simple" case folding mapping. The
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"simple" mapping was chosen because of a key convenient property: every
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"simple" mapping is a mapping from exactly one code point to exactly one other
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code point. This makes case insensitive matching of character classes, for
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example, straight-forward to implement.
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When case insensitive mode is enabled (e.g., `(?i)[a]` is equivalent to `a|A`),
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then all characters classes are case folded as well.
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## RL1.6 Line Boundaries
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[UTS#18 RL1.6](https://unicode.org/reports/tr18/#Line_Boundaries)
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The regex crate only provides support for recognizing the `\n` (`END OF LINE`)
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character as a line boundary. This choice was made mostly for implementation
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convenience, and to avoid performance cliffs that Unicode word boundaries are
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subject to.
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Ideally, it would be nice to at least support `\r\n` as a line boundary as
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well, and in theory, this could be done efficiently.
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## RL1.7 Code Points
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[UTS#18 RL1.7](https://unicode.org/reports/tr18/#Supplementary_Characters)
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The regex crate provides full support for Unicode code point matching. Namely,
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the fundamental atom of any match is always a single code point.
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Given Rust's strong ties to UTF-8, the following guarantees are also provided:
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* All matches are reported on valid UTF-8 code unit boundaries. That is, any
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match range returned by the public regex API is guaranteed to successfully
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slice the string that was searched.
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* By consequence of the above, it is impossible to match surrogode code points.
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No support for UTF-16 is provided, so this is never necessary.
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Note that when Unicode mode is disabled, the fundamental atom of matching is
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no longer a code point but a single byte. When Unicode mode is disabled, many
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Unicode features are disabled as well. For example, `(?-u)\pL` is not a valid
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regex but `\pL(?-u)\xFF` (matches any Unicode `Letter` followed by the literal
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byte `\xFF`) is, for example.
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