Unicode Text Segmentation for Go

This Go package implements Unicode Text Segmentation according to Unicode Standard Annex #29, Unicode Line Breaking according to Unicode Standard Annex #14 (Unicode version 14.0.0), and monospace font string width calculation similar to wcwidth.

Background

Grapheme Clusters

In Go, strings are read-only slices of bytes. They can be turned into Unicode code points using the for loop or by casting: []rune(str). However, multiple code points may be combined into one user-perceived character or what the Unicode specification calls "grapheme cluster". Here are some examples:

String	Bytes (UTF-8)	Code points (runes)	Grapheme clusters
Käse	6 bytes: `4b 61 cc 88 73 65`	5 code points: `4b 61 308 73 65`	4 clusters: `[4b],[61 308],[73],[65]`
🏳️‍🌈	14 bytes: `f0 9f 8f b3 ef b8 8f e2 80 8d f0 9f 8c 88`	4 code points: `1f3f3 fe0f 200d 1f308`	1 cluster: `[1f3f3 fe0f 200d 1f308]`
🇩🇪	8 bytes: `f0 9f 87 a9 f0 9f 87 aa`	2 code points: `1f1e9 1f1ea`	1 cluster: `[1f1e9 1f1ea]`

This package provides tools to iterate over these grapheme clusters. This may be used to determine the number of user-perceived characters, to split strings in their intended places, or to extract individual characters which form a unit.

Word Boundaries

Word boundaries are used in a number of different contexts. The most familiar ones are selection (double-click mouse selection), cursor movement ("move to next word" control-arrow keys), and the dialog option "Whole Word Search" for search and replace. They are also used in database queries, to determine whether elements are within a certain number of words of one another. Searching may also use word boundaries in determining matching items. This package provides tools to determine word boundaries within strings.

Sentence Boundaries

Sentence boundaries are often used for triple-click or some other method of selecting or iterating through blocks of text that are larger than single words. They are also used to determine whether words occur within the same sentence in database queries. This package provides tools to determine sentence boundaries within strings.

Line Breaking

Line breaking, also known as word wrapping, is the process of breaking a section of text into lines such that it will fit in the available width of a page, window or other display area. This package provides tools to determine where a string may or may not be broken and where it must be broken (for example after newline characters).

Monospace Width

Most terminals or text displays / text editors using a monospace font (for example source code editors) use a fixed width for each character. Some characters such as emojis or characters found in Asian and other languages may take up more than one character cell. This package provides tools to determine the number of cells a string will take up when displayed in a monospace font. See here for more information.

Installation

go get github.com/rivo/uniseg

Examples

Counting Characters in a String

n := uniseg.GraphemeClusterCount("🇩🇪🏳️‍🌈")
fmt.Println(n)
// 2

Calculating the Monospace String Width

width := uniseg.StringWidth("🇩🇪🏳️‍🌈!")
fmt.Println(width)
// 5

Using the `Graphemes` Class

This is the most convenient method of iterating over grapheme clusters:

gr := uniseg.NewGraphemes("👍🏼!")
for gr.Next() {
	fmt.Printf("%x ", gr.Runes())
}
// [1f44d 1f3fc] [21]

Using the `Step` or `StepString` Function

This is orders of magnitude faster than the Graphemes class, but it requires the handling of states and boundaries:

str := "🇩🇪🏳️‍🌈"
state := -1
var c string
for len(str) > 0 {
	c, str, _, state = uniseg.StepString(str, state)
	fmt.Printf("%x ", []rune(c))
}
// [1f1e9 1f1ea] [1f3f3 fe0f 200d 1f308]

Advanced Examples

Breaking into grapheme clusters and evaluating line breaks:

str := "First line.\nSecond line."
state := -1
var (
	c          string
	boundaries int
)
for len(str) > 0 {
	c, str, boundaries, state = uniseg.StepString(str, state)
	fmt.Print(c)
	if boundaries&uniseg.MaskLine == uniseg.LineCanBreak {
		fmt.Print("|")
	} else if boundaries&uniseg.MaskLine == uniseg.LineMustBreak {
		fmt.Print("‖")
	}
}
// First |line.
// ‖Second |line.‖

If you're only interested in word segmentation, use FirstWord or FirstWordInString:

str := "Hello, world!"
state := -1
var c string
for len(str) > 0 {
	c, str, state = uniseg.FirstWordInString(str, state)
	fmt.Printf("(%s)\n", c)
}
// (Hello)
// (,)
// ( )
// (world)
// (!)

Similarly, use

FirstGraphemeCluster or FirstGraphemeClusterInString for grapheme cluster determination only,
FirstSentence or FirstSentenceInString for sentence segmentation only, and
FirstLineSegment or FirstLineSegmentInString for line breaking / word wrapping (although using Step or StepString is preferred as it will observe grapheme cluster boundaries).

Finally, if you need to reverse a string while preserving grapheme clusters, use ReverseString:

fmt.Println(uniseg.ReverseString("🇩🇪🏳️‍🌈"))
// 🏳️‍🌈🇩🇪

Documentation

Refer to https://pkg.go.dev/github.com/rivo/uniseg for the package's documentation.

Dependencies

This package does not depend on any packages outside the standard library.

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ex-preman/uniseg