unicodeset

An implementation of UnicodeSet patterns for JavaScript. A UnicodeSet is a string syntax for succinctly defining a set of unicode characters (single codepoints) and strings (multiple codepoints). It is somewhat similar to regular expressions. In fact in current browsers, RegExp has preliminary support for a subset of UnicodeSet features. An example: "[[:alpha:] - [A-Z]]" gives all alphabetic characters excluding A through Z. The full UnicodeSet syntax is described below.

The UnicodeSet is greedily evaluated as a RangeGroup, using the range-group library, the type being UnicodeNormType. This gives you all the features of a RangeGroup, like the ability to search or randomly sample from the set. Since we evaluate greedily, note there are some limitations when dealing with strings (but not characters), as strings are multidimensional. These are described below.

This library is currently only available for node. Support for browsers is feasible, but would require some work to build a packed version of the unicode character/properties data. This library uses both raw unicode source files and node-unicode 15.0 to support unicode character names and properties. This library performs lazy loading of unicode data as it is needed.

API documentation | npm package | GitHub source code

Usage

For installation, you'll likely want to install range-group as well as that is what a UnicodeSet is evaluated to:

npm i unicodeset range-group

As the library currently only supports node, a bundled version is not provided.

Example usage:

import UnicodeSet from "unicodeset";
import { RangeGroup, Sampler } from "range-group";

// async, as it lazily loads unicode data as needed
const group = await UnicodeSet("[[:alpha:] - [A-Z]]");

// use the RangeGroup as you normally would
group.size();
group.iterate();
group.has("C");
const sampler = new Sampler(group);
sampler.sample();

This library also provides an extension RangeGroup.prototype.addRegenerate, which adds the ranges from a RangeGroup to a regenerate object. This can then be used to output a unicode aware RegExp from the resulting RangeGroup. The regenerate library is not included as a dependency, so will need to be added separately. Example:

import UnicodeSet from "unicodeset";
import regenerate from "regenerate";

const re = (await UnicodeSet("[[abcd] & [bcd]]"))
  .addRegenerate(regenerate())
  .toString();
// "[b-d]"

Some internal options and methods used for parsing and evaluating the pattern are available as well for those who want to hack something more custom together. E.g. You can get the parsed AST and evalute it as something other than a RangeGroup. Backwards compatibility will not be guaranted for these. Check the API documentation for details.

UnicodeSet syntax

Overview

The formal specification of a UnicodeSet can be found here, though I will aim to provide a clearer description here. I have also made some small, mostly backwards compatible enhancements for this implementation.

Some other helpful resources: The ICU has a C++/Java implementation, with further usage information documented here; their implementation will differ slightly from this one. There is also an online UnicodeSet utility, which uses the ICU Java implementation.

There are three ways to define a set for a UnicodeSet:

1. Enumerated: An enumerated list of characters/strings
2. Named: A set given by all the characters that match a unicode property
3. Composed: A set composed of the union, difference, intersection, or symmetric difference of several other sets

A set is surrounded with brackets, e.g. [a-zXYZ], [:letter:], or [[:letter:]-[abc]]. The exception is #2, which permits an alternative perl-like syntax, e.g. \p{letter}. Except where mentioned below, whitespace is permitted anywhere in the UnicodeSet string to help with readability.

For types #1 and #2, you use character literals to specify the characters, strings, property names, or property values inside the set. The following formats can be used:

For JavaScript string literals, you may consider using raw strings to avoid the double backslashes:

String.raw`\xFF` // same as "\\xFF"

The three syntax types are described in more detail below

#1 Enumerated

Characters can be listed individually: [abcXYZ0123].

Use a hyphen to indicate a range of characters: [a-z]. The first character's codepoint must be less than or equal to the second.

A string (multiple codepoints) is specified in curly braces: [{str5}].

Use a hyphen to indicate a range of strings: [{str0}-{str5}]. If the first string is longer, it indicates a shared prefix, e.g. [{str0}-{5}] is equivalent to [{str0}-{str5}]. However, the same does not apply in reverse: the second string must not be longer than the first. Ignoring any shared prefix, the corresponding codepoints from the first string must all be less than or equal to the codepoints of the second string. You can think of a string range as a nested character range. For example, the string range [{ab}-{cd}] is equivalent to [{ab}{ac}{ad}{bb}{bc}{bd}{cb}{cc}{cd}].

Limitation: UnicodeSets are greedily evaluated as a RangeGroup. As string ranges are technically multi-dimensional ranges, they are converted to one-dimensional ones via enumeration (see toRanges1d). For most cases this should be fine, but in others the memory requirements may be prohibitive.

The various definitions can be combined in a single set by concatenating them. The set will be the union of each: [abc A - Z {foo} {str0} - {str5}]. Note that whitespace can be inserted anywhere for readability.

You can invert a set by prefixing a hyphen - or caret ^: [^xyz], [-A-Z0-9].

Limitation: When inverting strings and string ranges, they are treated as one-dimensional. That means we only invert the final codepoint in the string, leaving the remaining codepoints (the prefix) unchanged.

A set can be empty. The inverted empty set ([-] or similar) has special meaning, indicating a range of all characters, equivalent to [\u{0}-\u{10FFFF}]; it doesn't include all strings.

Reserved characters:

• A hyphen - or caret ^ that appears as the first non-whitespace character
• One of [, \p, or \P that appear first after whitespace and inverting character. These mark the start of composed syntax.
• Inside a string range, a closing curly brace }
• Outside a string range, a hyphen -, opening curly brace {, or closing bracket ]

#2 Named

Most, but not all unicode properties are supported. The list of property names and values has been extracted from the raw unicode source. Property values for some are not as easily extracted, so are not included. For the actual codepoints, this library is using the data provided by the node-unicode 15.0 library. This library has pretty comprehensive support, but some of the less common like numeric properties are not included. An error will be thrown if a unicode property is unknown, or there is no codepoint data available for it. (If you find the current support lacking and are interested in contributing, I have a simple strategy outlined for getting comprehensive property support; contact me if interested)

Following the unicode matching rules, whitespace, underscores, medial hyphens, and casing are all insignificant for the property names and values.

There are two forms allowed:

1. Posix style: [:general_category=letter:]. Whitespace is not allowed between the colon and bracket.
2. Perl style: \p{general_category=letter}

For binary properties the value defaults to true if not given: [:whitespace:] is equivalent to [:whitespace=true:].

You may also omit the property name, which will default to general_category (first priority) or script (second priority): [:letter:], [:latin:]

In all other cases, both property name and value must be provided.

You can invert the set:

• You can prefix with a caret ^: [:^letter:] or \p{^letter}
• You can use a not-equal character ≠: [:gc≠letter:].
• For Perl style, you can use a capital P: \P{letter}
• For binary properties, inverting is equivalent to negating the property value: [:wspace=false:] is equivalent to [:^wspace:].

While not recommended, you can combine invert syntaxes for multiple negation (two inverts cancel eachother): \P{^wspace≠false}.

Reserved characters:

• As the first character, a caret ^.
• For property names, equal = and not-equal ≠
• For Posix style, a colon :
• For Perl style, a closing curly brace }

These should not be a big concern, as they're not valid characters for a unicode property name anyways.

#3 Composed

Combine sets with a set operator: [[a-z] & [a-f] - [bc] + [0-9]]. Set operations have left-to-right operator precedence. You can nest multiple composed sets to perform grouping and control the operator precedence more explicitly.

Four set operations are allowed:

• Union, + operator; also the default if no operator character is used: [[abc][cde]] or [[abc]+[cde]] are equivalent to [abcde]
• Difference, - operator: [[abc]-[cde]] is equivalent to [ab]
• Intersection, & operator: [[abc]&[cde] is equivalent to [c]
• Symmetric difference, ~ operator: [[abc]~[cde]] is equivalent to [abde]

As with the Enumerated syntax, a composed set can be inverted by prefixing with a hyphen - or caret ^: [^[abc] & [cde]].

Technically, the original specification allows enumerated sets without brackets when performing a union. For example [abc [:whitespace:]] instead of [[abc][:whitespace:]]. For this implementation, I'm forcing enumerated sets to always be enclosed in brackets for clarity and consistency.