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Programming-Idioms

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Idiom #344 Extract filename extension

Assign to ext the fragment of the string f after the last dot character, or the empty string if f does not contain a dot.

E.g. "photo.jpg" -> "jpg"

ext must not contain the dot character.

import "path/filepath"
import "strings"
ext := strings.TrimPrefix(filepath.Ext(f), ".")

filepath.Ext returns a string including the final dot of f. We remove this dot with strings.TrimPrefix.
New implementation...
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programming-idioms.org 
ext := strings.TrimPrefix(filepath.Ext(f), ".")
int i = f.lastIndexOf('.');
String ext = i != -1 ? f.substring(++i) : "";
ext := ExtractFileExt(f);
if (ext <> '') then system.delete(ext,1,1);
ext = os.path.splitext(f)[1][1:]
ext = File.extname(f).delete_prefix(".")