Question about strings.EqualFold vs Python's casefold

[Miki Tebeka]

Hi,

I'm trying to find an example where strings.EqualFold returns true but comparison of strings.ToLower fails.

I've found this example (in Python):

s1 = "der Fluß"

s2 = "der Fluss"

print('lower', s1.lower() == s2.lower())

print('fold ', s1.casefold() == s2.casefold())

Which prints False for lower and True for casefold.

When I try the same in Go

package main

import (

"fmt"

"strings"

)

func main() {

s1 := "der Fluß"

s2 := "der Fluss"

fmt.Println("lower", strings.ToLower(s1) == strings.ToLower(s2))

fmt.Println("fold ", strings.EqualFold(s1, s2))

}

I get false for both ToLower and EqualFold.

Shouldn't Unicode folding be the same across languages?

Also, does anyone have an example I can show in Go where ToLower does not compare and EqualFold does?

Thanks,

Miki

[Ian Lance Taylor]

strings.EqualFold uses Unicode case folding, not case mapping. Case
folding is only one-to-one character transformations. Converting "ss"
to "ß" is case mapping, and as such is not used by strings.EqualFold.
For that, you want the x/text/search package, as in

package main

import (
"fmt"

"golang.org/x/text/language"
"golang.org/x/text/search"
)

func main() {
m := search.New(language.German, search.Loose)

s1 := "der Fluß"
s2 := "der Fluss"

fmt.Println(m.EqualString(s1, s2))
}

That should print true.

An example where strings.EqualFold does not return the same result as
strings.ToLower(s1) == strings.ToLower(s2) is

s1 := "σς"
s2 := "ΣΣ"

strings.EqualFold will return true but comparing the ToLower of the
strings will return false. This is because there are two lower case
forms of Σ (depending on position in the word), but strings.ToLower
can of course only return one.

Ian

[Miki Tebeka]

Thanks!