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Unicode conformance

This document describes the regex crate's conformance to Unicode's UTS#18 report, which lays out 3 levels of support: Basic, Extended and Tailored.

Full support for Level 1 ("Basic Unicode Support") is provided with two exceptions:

Line boundaries are not Unicode aware. Namely, only the \n (END OF LINE) character is recognized as a line boundary.
The compatibility properties specified by RL1.2a are ASCII-only definitions.

Little to no support is provided for either Level 2 or Level 3. For the most part, this is because the features are either complex/hard to implement, or at the very least, very difficult to implement without sacrificing performance. For example, tackling canonical equivalence such that matching worked as one would expect regardless of normalization form would be a significant undertaking. This is at least partially a result of the fact that this regex engine is based on finite automata, which admits less flexibility normally associated with backtracking implementations.

RL1.1 Hex Notation

UTS#18 RL1.1

Hex Notation refers to the ability to specify a Unicode code point in a regular expression via its hexadecimal code point representation. This is useful in environments that have poor Unicode font rendering or if you need to express a code point that is not normally displayable. All forms of hexadecimal notation are supported

\x7F        hex character code (exactly two digits)
\x{10FFFF}  any hex character code corresponding to a Unicode code point
\u007F      hex character code (exactly four digits)
\u{7F}      any hex character code corresponding to a Unicode code point
\U0000007F  hex character code (exactly eight digits)
\U{7F}      any hex character code corresponding to a Unicode code point

Briefly, the \x{...}, \u{...} and \U{...} are all exactly equivalent ways of expressing hexadecimal code points. Any number of digits can be written within the brackets. In contrast, \xNN, \uNNNN, \UNNNNNNNN are all fixed-width variants of the same idea.

Note that when Unicode mode is disabled, any non-ASCII Unicode codepoint is banned. Additionally, the \xNN syntax represents arbitrary bytes when Unicode mode is disabled. That is, the regex \xFF matches the Unicode codepoint U+00FF (encoded as \xC3\xBF in UTF-8) while the regex (?-u)\xFF matches the literal byte \xFF.

RL1.2 Properties

UTS#18 RL1.2

Full support for Unicode property syntax is provided. Unicode properties provide a convenient way to construct character classes of groups of code points specified by Unicode. The regex crate does not provide exhaustive support, but covers a useful subset. In particular:

General categories
Scripts and Script Extensions
Age
A smattering of boolean properties, including all of those specified by RL1.2 explicitly.

In all cases, property name and value abbreviations are supported, and all names/values are matched loosely without regard for case, whitespace or underscores. Property name aliases can be found in Unicode's PropertyAliases.txt file, while property value aliases can be found in Unicode's PropertyValueAliases.txt file.

The syntax supported is also consistent with the UTS#18 recommendation:

\p{Greek} selects the Greek script. Equivalent expressions follow: \p{sc:Greek}, \p{Script:Greek}, \p{Sc=Greek}, \p{script=Greek}, \P{sc!=Greek}. Similarly for General_Category (or gc for short) and Script_Extensions (or scx for short).
\p{age:3.2} selects all code points in Unicode 3.2.
\p{Alphabetic} selects the "alphabetic" property and can be abbreviated via \p{alpha} (for example).
Single letter variants for properties with single letter abbreviations. For example, \p{Letter} can be equivalently written as \pL.

The following is a list of all properties supported by the regex crate (starred properties correspond to properties required by RL1.2):

General_Category * (including Any, ASCII and Assigned)
Script *
Script_Extensions *
Age
ASCII_Hex_Digit
Alphabetic *
Bidi_Control
Case_Ignorable
Cased
Changes_When_Casefolded
Changes_When_Casemapped
Changes_When_Lowercased
Changes_When_Titlecased
Changes_When_Uppercased
Dash
Default_Ignorable_Code_Point *
Deprecated
Diacritic
Emoji
Emoji_Presentation
Emoji_Modifier
Emoji_Modifier_Base
Emoji_Component
Extended_Pictographic
Extender
Grapheme_Base
Grapheme_Cluster_Break
Grapheme_Extend
Hex_Digit
IDS_Binary_Operator
IDS_Trinary_Operator
ID_Continue
ID_Start
Join_Control
Logical_Order_Exception
Lowercase *
Math
Noncharacter_Code_Point *
Pattern_Syntax
Pattern_White_Space
Prepended_Concatenation_Mark
Quotation_Mark
Radical
Regional_Indicator
Sentence_Break
Sentence_Terminal
Soft_Dotted
Terminal_Punctuation
Unified_Ideograph
Uppercase *
Variation_Selector
White_Space *
Word_Break
XID_Continue
XID_Start

RL1.2a Compatibility Properties

UTS#18 RL1.2a

The regex crate only provides ASCII definitions of the compatibility properties documented in UTS#18 Annex C (sans the \X class, for matching grapheme clusters, which isn't provided at all). This is because it seems to be consistent with most other regular expression engines, and in particular, because these are often referred to as "ASCII" or "POSIX" character classes.

Note that the \w, \s and \d character classes are Unicode aware. Their traditional ASCII definition can be used by disabling Unicode. That is, [[:word:]] and (?-u)\w are equivalent.

RL1.3 Subtraction and Intersection

UTS#18 RL1.3

The regex crate provides full support for nested character classes, along with union, intersection (&&), difference (--) and symmetric difference (~~) operations on arbitrary character classes.

For example, to match all non-ASCII letters, you could use either [\p{Letter}--\p{Ascii}] (difference) or [\p{Letter}&&[^\p{Ascii}]] (intersecting the negation).

RL1.4 Simple Word Boundaries

UTS#18 RL1.4

The regex crate provides basic Unicode aware word boundary assertions. A word boundary assertion can be written as \b, or \B as its negation. A word boundary negation corresponds to a zero-width match, where its adjacent characters correspond to word and non-word, or non-word and word characters.

Conformance in this case chooses to define word character in the same way that the \w character class is defined: a code point that is a member of one of the following classes:

\p{Alphabetic}
\p{Join_Control}
\p{gc:Mark}
\p{gc:Decimal_Number}
\p{gc:Connector_Punctuation}

In particular, this differs slightly from the prescription given in RL1.4 but is permissible according to UTS#18 Annex C. Namely, it is convenient and simpler to have \w and \b be in sync with one another.

Finally, Unicode word boundaries can be disabled, which will cause ASCII word boundaries to be used instead. That is, \b is a Unicode word boundary while (?-u)\b is an ASCII-only word boundary. This can occasionally be beneficial if performance is important, since the implementation of Unicode word boundaries is currently sub-optimal on non-ASCII text.

RL1.5 Simple Loose Matches

UTS#18 RL1.5

The regex crate provides full support for case insensitive matching in accordance with RL1.5. That is, it uses the "simple" case folding mapping. The "simple" mapping was chosen because of a key convenient property: every "simple" mapping is a mapping from exactly one code point to exactly one other code point. This makes case insensitive matching of character classes, for example, straight-forward to implement.

When case insensitive mode is enabled (e.g., (?i)[a] is equivalent to a|A), then all characters classes are case folded as well.

RL1.6 Line Boundaries

UTS#18 RL1.6

The regex crate only provides support for recognizing the \n (END OF LINE) character as a line boundary. This choice was made mostly for implementation convenience, and to avoid performance cliffs that Unicode word boundaries are subject to.

Ideally, it would be nice to at least support \r\n as a line boundary as well, and in theory, this could be done efficiently.

RL1.7 Code Points

UTS#18 RL1.7

The regex crate provides full support for Unicode code point matching. Namely, the fundamental atom of any match is always a single code point.

Given Rust's strong ties to UTF-8, the following guarantees are also provided:

All matches are reported on valid UTF-8 code unit boundaries. That is, any match range returned by the public regex API is guaranteed to successfully slice the string that was searched.
By consequence of the above, it is impossible to match surrogode code points. No support for UTF-16 is provided, so this is never necessary.

Note that when Unicode mode is disabled, the fundamental atom of matching is no longer a code point but a single byte. When Unicode mode is disabled, many Unicode features are disabled as well. For example, (?-u)\pL is not a valid regex but \pL(?-u)\xFF (matches any Unicode Letter followed by the literal byte \xFF) is, for example.

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