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import "github.com/rivo/uniseg"
Package uniseg implements Unicode Text Segmentation, Unicode Line Breaking, and string width calculation for monospace fonts. Unicode Text Segmentation conforms to Unicode Standard Annex #29 (https://unicode.org/reports/tr29/) and Unicode Line Breaking conforms to Unicode Standard Annex #14 (https://unicode.org/reports/tr14/).
In short, using this package, you can split a string into grapheme clusters (what people would usually refer to as a "character"), into words, and into sentences. Or, in its simplest case, this package allows you to count the number of characters in a string, especially when it contains complex characters such as emojis, combining characters, or characters from Asian, Arabic, Hebrew, or other languages. Additionally, you can use it to implement line breaking (or "word wrapping"), that is, to determine where text can be broken over to the next line when the width of the line is not big enough to fit the entire text. Finally, you can use it to calculate the display width of a string for monospace fonts.
If you just want to count the number of characters in a string, you can use GraphemeClusterCount. If you want to determine the display width of a string, you can use StringWidth. If you want to iterate over a string, you can use Step, StepString, or the Graphemes class (more convenient but less performant). This will provide you with all information: grapheme clusters, word boundaries, sentence boundaries, line breaks, and monospace character widths. The specialized functions FirstGraphemeCluster, FirstGraphemeClusterInString, FirstWord, FirstWordInString, FirstSentence, and FirstSentenceInString can be used if only one type of information is needed.
Consider the rainbow flag emoji: π³οΈβπ. On most modern systems, it appears as one character. But its string representation actually has 14 bytes, so counting bytes (or using len("π³οΈβπ")) will not work as expected. Counting runes won't, either: The flag has 4 Unicode code points, thus 4 runes. The stdlib function utf8.RuneCountInString("π³οΈβπ") and len([]rune("π³οΈβπ")) will both return 4.
The GraphemeClusterCount function will return 1 for the rainbow flag emoji. The Graphemes class and a variety of functions in this package will allow you to split strings into its grapheme clusters.
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. This package provides methods for determining word 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 methods for determining sentence boundaries.
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 methods to determine the positions in a string where a line must be broken, may be broken, or must not be broken.
Monospace width, as referred to in this package, is the width of a string in a monospace font. This is commonly used in terminal user interfaces or text displays or editors that don't support proportional fonts. A width of 1 corresponds to a single character cell. The C function wcswidth() and its implementation in other programming languages is in widespread use for the same purpose. However, there is no standard for the calculation of such widths, and this package differs from wcswidth() in a number of ways, presumably to generate more visually pleasing results.
To start, we assume that every code point has a width of 1, with the following exceptions:
For Hangul grapheme clusters composed of conjoining Jamo and for Regional Indicators (flags), all code points except the first one have a width of 0. For grapheme clusters starting with an Extended Pictographic, any additional code point will force a total width of 2, except if the Variation Selector-15 (U+FE0E) is included, in which case the total width is always 1. Grapheme clusters ending with Variation Selector-16 (U+FE0F) have a width of 2.
Note that whether these widths appear correct depends on your application's render engine, to which extent it conforms to the Unicode Standard, and its choice of font.
const ( LineDontBreak = iota // You may not break the line here. LineCanBreak // You may or may not break the line here. LineMustBreak // You must break the line here. )
These constants define whether a given text may be broken into the next line. If the break is optional (LineCanBreak), you may choose to break or not based on your own criteria, for example, if the text has reached the available width.
const ( MaskLine = 3 MaskWord = 4 MaskSentence = 8 )
The bit masks used to extract boundary information returned by Step.
const ShiftWidth = 4
The number of bits to shift the boundary information returned by Step to obtain the monospace width of the grapheme cluster.
var EastAsianAmbiguousWidth = 1
EastAsianAmbiguousWidth specifies the monospace width for East Asian characters classified as Ambiguous. The default is 1 but some rare fonts render them with a width of 2.
func FirstGraphemeCluster(b []byte, state int) (cluster, rest []byte, width, newState int)
FirstGraphemeCluster returns the first grapheme cluster found in the given byte slice according to the rules of Unicode Standard Annex #29, Grapheme Cluster Boundaries. This function can be called continuously to extract all grapheme clusters from a byte slice, as illustrated in the example below.
If you don't know the current state, for example when calling the function for the first time, you must pass -1. For consecutive calls, pass the state and rest slice returned by the previous call.
The "rest" slice is the sub-slice of the original byte slice "b" starting after the last byte of the identified grapheme cluster. If the length of the "rest" slice is 0, the entire byte slice "b" has been processed. The "cluster" byte slice is the sub-slice of the input slice containing the identified grapheme cluster.
The returned width is the width of the grapheme cluster for most monospace fonts where a value of 1 represents one character cell.
Given an empty byte slice "b", the function returns nil values.
While slightly less convenient than using the Graphemes class, this function has much better performance and makes no allocations. It lends itself well to large byte slices.
Code:
package main
import (
"fmt"
"github.com/rivo/uniseg"
)
func main() {
b := []byte("π©πͺπ³οΈβπ!")
state := -1
var c []byte
for len(b) > 0 {
var width int
c, b, width, state = uniseg.FirstGraphemeCluster(b, state)
fmt.Println(string(c), width)
}
}
Output:
π©πͺ 2 π³οΈβπ 2 ! 1
func FirstGraphemeClusterInString(str string, state int) (cluster, rest string, width, newState int)
FirstGraphemeClusterInString is like FirstGraphemeCluster but its input and outputs are strings.
Code:
package main
import (
"fmt"
"github.com/rivo/uniseg"
)
func main() {
str := "π©πͺπ³οΈβπ!"
state := -1
var c string
for len(str) > 0 {
var width int
c, str, width, state = uniseg.FirstGraphemeClusterInString(str, state)
fmt.Println(c, width)
}
}
Output:
π©πͺ 2 π³οΈβπ 2 ! 1
func FirstLineSegment(b []byte, state int) (segment, rest []byte, mustBreak bool, newState int)
FirstLineSegment returns the prefix of the given byte slice after which a decision to break the string over to the next line can or must be made, according to the rules of Unicode Standard Annex #14. This is used to implement 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.
The returned "segment" may not be broken into smaller parts, unless no other breaking opportunities present themselves, in which case you may break by grapheme clusters (using the FirstGraphemeCluster function to determine the grapheme clusters).
The "mustBreak" flag indicates whether you MUST break the line after the given segment (true), for example after newline characters, or you MAY break the line after the given segment (false).
This function can be called continuously to extract all non-breaking sub-sets from a byte slice, as illustrated in the example below.
If you don't know the current state, for example when calling the function for the first time, you must pass -1. For consecutive calls, pass the state and rest slice returned by the previous call.
The "rest" slice is the sub-slice of the original byte slice "b" starting after the last byte of the identified line segment. If the length of the "rest" slice is 0, the entire byte slice "b" has been processed. The "segment" byte slice is the sub-slice of the input slice containing the identified line segment.
Given an empty byte slice "b", the function returns nil values.
Note that in accordance with UAX #14 LB3, the final segment will end with "mustBreak" set to true. You can choose to ignore this by checking if the length of the "rest" slice is 0 and calling HasTrailingLineBreak or HasTrailingLineBreakInString on the last rune.
Note also that this algorithm may break within grapheme clusters. This is addressed in Section 8.2 Example 6 of UAX #14. To avoid this, you can use the Step function instead.
Code:
package main
import (
"fmt"
"github.com/rivo/uniseg"
)
func main() {
b := []byte("First line.\nSecond line.")
state := -1
var (
c []byte
mustBreak bool
)
for len(b) > 0 {
c, b, mustBreak, state = uniseg.FirstLineSegment(b, state)
fmt.Printf("(%s)", string(c))
if mustBreak {
fmt.Print("!")
}
}
}
Output:
(First )(line. )!(Second )(line.)!
func FirstLineSegmentInString(str string, state int) (segment, rest string, mustBreak bool, newState int)
FirstLineSegmentInString is like FirstLineSegment but its input and outputs are strings.
Code:
package main
import (
"fmt"
"github.com/rivo/uniseg"
)
func main() {
str := "First line.\nSecond line."
state := -1
var (
c string
mustBreak bool
)
for len(str) > 0 {
c, str, mustBreak, state = uniseg.FirstLineSegmentInString(str, state)
fmt.Printf("(%s)", c)
if mustBreak {
fmt.Println(" < must break")
} else {
fmt.Println(" < may break")
}
}
}
Output:
(First ) < may break (line. ) < must break (Second ) < may break (line.) < must break
func FirstSentence(b []byte, state int) (sentence, rest []byte, newState int)
FirstSentence returns the first sentence found in the given byte slice according to the rules of Unicode Standard Annex #29, Sentence Boundaries. This function can be called continuously to extract all sentences from a byte slice, as illustrated in the example below.
If you don't know the current state, for example when calling the function for the first time, you must pass -1. For consecutive calls, pass the state and rest slice returned by the previous call.
The "rest" slice is the sub-slice of the original byte slice "b" starting after the last byte of the identified sentence. If the length of the "rest" slice is 0, the entire byte slice "b" has been processed. The "sentence" byte slice is the sub-slice of the input slice containing the identified sentence.
Given an empty byte slice "b", the function returns nil values.
Code:
package main
import (
"fmt"
"github.com/rivo/uniseg"
)
func main() {
b := []byte("This is sentence 1.0. And this is sentence two.")
state := -1
var c []byte
for len(b) > 0 {
c, b, state = uniseg.FirstSentence(b, state)
fmt.Printf("(%s)\n", string(c))
}
}
Output:
(This is sentence 1.0. ) (And this is sentence two.)
func FirstSentenceInString(str string, state int) (sentence, rest string, newState int)
FirstSentenceInString is like FirstSentence but its input and outputs are strings.
Code:
package main
import (
"fmt"
"github.com/rivo/uniseg"
)
func main() {
str := "This is sentence 1.0. And this is sentence two."
state := -1
var c string
for len(str) > 0 {
c, str, state = uniseg.FirstSentenceInString(str, state)
fmt.Printf("(%s)\n", c)
}
}
Output:
(This is sentence 1.0. ) (And this is sentence two.)
func FirstWord(b []byte, state int) (word, rest []byte, newState int)
FirstWord returns the first word found in the given byte slice according to the rules of Unicode Standard Annex #29, Word Boundaries. This function can be called continuously to extract all words from a byte slice, as illustrated in the example below.
If you don't know the current state, for example when calling the function for the first time, you must pass -1. For consecutive calls, pass the state and rest slice returned by the previous call.
The "rest" slice is the sub-slice of the original byte slice "b" starting after the last byte of the identified word. If the length of the "rest" slice is 0, the entire byte slice "b" has been processed. The "word" byte slice is the sub-slice of the input slice containing the identified word.
Given an empty byte slice "b", the function returns nil values.
Code:
package main
import (
"fmt"
"github.com/rivo/uniseg"
)
func main() {
b := []byte("Hello, world!")
state := -1
var c []byte
for len(b) > 0 {
c, b, state = uniseg.FirstWord(b, state)
fmt.Printf("(%s)\n", string(c))
}
}
Output:
(Hello) (,) ( ) (world) (!)
func FirstWordInString(str string, state int) (word, rest string, newState int)
FirstWordInString is like FirstWord but its input and outputs are strings.
Code:
package main
import (
"fmt"
"github.com/rivo/uniseg"
)
func main() {
str := "Hello, world!"
state := -1
var c string
for len(str) > 0 {
c, str, state = uniseg.FirstWordInString(str, state)
fmt.Printf("(%s)\n", c)
}
}
Output:
(Hello) (,) ( ) (world) (!)
func GraphemeClusterCount(s string) (n int)
GraphemeClusterCount returns the number of user-perceived characters (grapheme clusters) for the given string.
Code:
package main
import (
"fmt"
"github.com/rivo/uniseg"
)
func main() {
n := uniseg.GraphemeClusterCount("π©πͺπ³οΈβπ")
fmt.Println(n)
}
Output:
2
func HasTrailingLineBreak(b []byte) bool
HasTrailingLineBreak returns true if the last rune in the given byte slice is one of the hard line break code points defined in LB4 and LB5 of UAX #14.
func HasTrailingLineBreakInString(str string) bool
HasTrailingLineBreakInString is like HasTrailingLineBreak but for a string.
func ReverseString(s string) string
ReverseString reverses the given string while observing grapheme cluster boundaries.
func Step(b []byte, state int) (cluster, rest []byte, boundaries int, newState int)
Step returns the first grapheme cluster (user-perceived character) found in the given byte slice. It also returns information about the boundary between that grapheme cluster and the one following it as well as the monospace width of the grapheme cluster. There are three types of boundary information: word boundaries, sentence boundaries, and line breaks. This function is therefore a combination of FirstGraphemeCluster, FirstWord, FirstSentence, and FirstLineSegment.
The "boundaries" return value can be evaluated as follows:
This function can be called continuously to extract all grapheme clusters from a byte slice, as illustrated in the examples below.
If you don't know which state to pass, for example when calling the function for the first time, you must pass -1. For consecutive calls, pass the state and rest slice returned by the previous call.
The "rest" slice is the sub-slice of the original byte slice "b" starting after the last byte of the identified grapheme cluster. If the length of the "rest" slice is 0, the entire byte slice "b" has been processed. The "cluster" byte slice is the sub-slice of the input slice containing the first identified grapheme cluster.
Given an empty byte slice "b", the function returns nil values.
While slightly less convenient than using the Graphemes class, this function has much better performance and makes no allocations. It lends itself well to large byte slices.
Note that in accordance with UAX #14 LB3, the final segment will end with a mandatory line break (boundaries&MaskLine == LineMustBreak). You can choose to ignore this by checking if the length of the "rest" slice is 0 and calling HasTrailingLineBreak or HasTrailingLineBreakInString on the last rune.
Code:
package main
import (
"fmt"
"github.com/rivo/uniseg"
)
func main() {
b := []byte("π©πͺπ³οΈβπ!")
state := -1
var c []byte
for len(b) > 0 {
var boundaries int
c, b, boundaries, state = uniseg.Step(b, state)
fmt.Println(string(c), boundaries>>uniseg.ShiftWidth)
}
}
Output:
π©πͺ 2 π³οΈβπ 2 ! 1
Code:
package main
import (
"fmt"
"github.com/rivo/uniseg"
)
func main() {
b := []byte("First line.\nSecond line.")
state := -1
var (
c []byte
boundaries int
)
for len(b) > 0 {
c, b, boundaries, state = uniseg.Step(b, state)
fmt.Print(string(c))
if boundaries&uniseg.MaskLine == uniseg.LineCanBreak {
fmt.Print("|")
} else if boundaries&uniseg.MaskLine == uniseg.LineMustBreak {
fmt.Print("β")
}
}
}
Output:
First |line. βSecond |line.β
Code:
package main
import (
"fmt"
"github.com/rivo/uniseg"
)
func main() {
b := []byte("This is sentence 1.0. And this is sentence two.")
state := -1
var (
c []byte
boundaries int
)
for len(b) > 0 {
c, b, boundaries, state = uniseg.Step(b, state)
fmt.Print(string(c))
if boundaries&uniseg.MaskSentence != 0 {
fmt.Print("|")
}
}
}
Output:
This is sentence 1.0. |And this is sentence two.|
Code:
package main
import (
"fmt"
"github.com/rivo/uniseg"
)
func main() {
b := []byte("Hello, world!")
state := -1
var (
c []byte
boundaries int
)
for len(b) > 0 {
c, b, boundaries, state = uniseg.Step(b, state)
fmt.Print(string(c))
if boundaries&uniseg.MaskWord != 0 {
fmt.Print("|")
}
}
}
Output:
Hello|,| |world|!|
func StepString(str string, state int) (cluster, rest string, boundaries int, newState int)
StepString is like Step but its input and outputs are strings.
Code:
package main
import (
"fmt"
"github.com/rivo/uniseg"
)
func main() {
str := "π©πͺπ³οΈβπ!"
state := -1
var c string
for len(str) > 0 {
var boundaries int
c, str, boundaries, state = uniseg.StepString(str, state)
fmt.Println(c, boundaries>>uniseg.ShiftWidth)
}
}
Output:
π©πͺ 2 π³οΈβπ 2 ! 1
Code:
package main
import (
"fmt"
"github.com/rivo/uniseg"
)
func main() {
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("β")
}
}
}
Output:
First |line. βSecond |line.β
Code:
package main
import (
"fmt"
"github.com/rivo/uniseg"
)
func main() {
str := "This is sentence 1.0. And this is sentence two."
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.MaskSentence != 0 {
fmt.Print("|")
}
}
}
Output:
This is sentence 1.0. |And this is sentence two.|
Code:
package main
import (
"fmt"
"github.com/rivo/uniseg"
)
func main() {
str := "Hello, world!"
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.MaskWord != 0 {
fmt.Print("|")
}
}
}
Output:
Hello|,| |world|!|
func StringWidth(s string) (width int)
StringWidth returns the monospace width for the given string, that is, the number of same-size cells to be occupied by the string.
Code:
package main
import (
"fmt"
"github.com/rivo/uniseg"
)
func main() {
fmt.Println(uniseg.StringWidth("Hello, δΈη"))
}
Output:
11
type Graphemes struct {
// contains filtered or unexported fields
}
Graphemes implements an iterator over Unicode grapheme clusters, or user-perceived characters. While iterating, it also provides information about word boundaries, sentence boundaries, line breaks, and monospace character widths.
After constructing the class via NewGraphemes for a given string "str", Graphemes.Next is called for every grapheme cluster in a loop until it returns false. Inside the loop, information about the grapheme cluster as well as boundary information and character width is available via the various methods (see examples below).
This class basically wraps the StepString parser and provides a convenient interface to it. If you are only interested in some parts of this package's functionality, using the specialized functions starting with "First" is almost always faster.
Code:
package main
import (
"fmt"
"github.com/rivo/uniseg"
)
func main() {
g := uniseg.NewGraphemes("π©πͺπ³οΈβπ")
for g.Next() {
fmt.Println(g.Str())
}
}
Output:
π©πͺ π³οΈβπ
Code:
package main
import (
"fmt"
"github.com/rivo/uniseg"
)
func main() {
g := uniseg.NewGraphemes("First line.\nSecond line.")
for g.Next() {
fmt.Print(g.Str())
if g.LineBreak() == uniseg.LineCanBreak {
fmt.Print("|")
} else if g.LineBreak() == uniseg.LineMustBreak {
fmt.Print("β")
}
}
if g.LineBreak() == uniseg.LineMustBreak {
fmt.Print("\nNo clusters left. LineMustBreak")
}
g.Reset()
if g.LineBreak() == uniseg.LineDontBreak {
fmt.Print("\nIterator has been reset. LineDontBreak")
}
}
Output:
First |line. βSecond |line.β No clusters left. LineMustBreak Iterator has been reset. LineDontBreak
Code:
package main
import (
"fmt"
"github.com/rivo/uniseg"
)
func main() {
g := uniseg.NewGraphemes("This is sentence 1.0. And this is sentence two.")
for g.Next() {
fmt.Print(g.Str())
if g.IsSentenceBoundary() {
fmt.Print("|")
}
}
}
Output:
This is sentence 1.0. |And this is sentence two.|
Code:
package main
import (
"fmt"
"github.com/rivo/uniseg"
)
func main() {
g := uniseg.NewGraphemes("Hello, world!")
for g.Next() {
fmt.Print(g.Str())
if g.IsWordBoundary() {
fmt.Print("|")
}
}
}
Output:
Hello|,| |world|!|
func NewGraphemes(str string) *Graphemes
NewGraphemes returns a new grapheme cluster iterator.
func (g *Graphemes) Bytes() []byte
Bytes returns a byte slice which corresponds to the current grapheme cluster. If the iterator is already past the end or Graphemes.Next has not yet been called, nil is returned.
func (g *Graphemes) IsSentenceBoundary() bool
IsSentenceBoundary returns true if a sentence ends after the current grapheme cluster.
func (g *Graphemes) IsWordBoundary() bool
IsWordBoundary returns true if a word ends after the current grapheme cluster.
func (g *Graphemes) LineBreak() int
LineBreak returns whether the line can be broken after the current grapheme cluster. A value of LineDontBreak means the line may not be broken, a value of LineMustBreak means the line must be broken, and a value of LineCanBreak means the line may or may not be broken.
func (g *Graphemes) Next() bool
Next advances the iterator by one grapheme cluster and returns false if no clusters are left. This function must be called before the first cluster is accessed.
func (g *Graphemes) Positions() (int, int)
Positions returns the interval of the current grapheme cluster as byte positions into the original string. The first returned value "from" indexes the first byte and the second returned value "to" indexes the first byte that is not included anymore, i.e. str[from:to] is the current grapheme cluster of the original string "str". If Graphemes.Next has not yet been called, both values are 0. If the iterator is already past the end, both values are 1.
func (g *Graphemes) Reset()
Reset puts the iterator into its initial state such that the next call to Graphemes.Next sets it to the first grapheme cluster again.
func (g *Graphemes) Runes() []rune
Runes returns a slice of runes (code points) which corresponds to the current grapheme cluster. If the iterator is already past the end or Graphemes.Next has not yet been called, nil is returned.
func (g *Graphemes) Str() string
Str returns a substring of the original string which corresponds to the current grapheme cluster. If the iterator is already past the end or Graphemes.Next has not yet been called, an empty string is returned.
func (g *Graphemes) Width() int
Width returns the monospace width of the current grapheme cluster.