Add preserveCharacters option (#70)

Co-authored-by: Sindre Sorhus <sindresorhus@gmail.com>

index.d.ts

@@ -132,6 +132,25 @@ export interface Options {
 	```
 	 */
 	readonly preserveTrailingDash?: boolean;
+
+	/**
+	Preserve certain characters.
+
+	It cannot contain the `separator`.
+
+	For example, if you want to slugify URLs, but preserve the HTML fragment `#` character, you could set `preserveCharacters: ['#']`.
+
+	@default []
+
+	@example
+	```
+	import slugify from '@sindresorhus/slugify';
+
+	slugify('foo_bar#baz', {preserveCharacters: ['#']});
+	//=> 'foo-bar#baz'
+	```
+	*/
+	readonly preserveCharacters?: string[];
 }
 
 /**

index.js

@@ -22,6 +22,23 @@ const removeMootSeparators = (string, separator) => {
 		.replace(new RegExp(`^${escapedSeparator}|${escapedSeparator}$`, 'g'), '');
 };
 
+const buildPatternSlug = options => {
+	let negationSetPattern = 'a-z\\d';
+	negationSetPattern += options.lowercase ? '' : 'A-Z';
+
+	if (options.preserveCharacters.length > 0) {
+		for (const character of options.preserveCharacters) {
+			if (character === options.separator) {
+				throw new Error(`The separator character \`${options.separator}\` cannot be included in preserved characters: ${options.preserveCharacters}`);
+			}
+
+			negationSetPattern += escapeStringRegexp(character);
+		}
+	}
+
+	return new RegExp(`[^${negationSetPattern}]+`, 'g');
+};
+
 export default function slugify(string, options) {
 	if (typeof string !== 'string') {
 		throw new TypeError(`Expected a string, got \`${typeof string}\``);
@@ -34,6 +51,7 @@ export default function slugify(string, options) {
 		customReplacements: [],
 		preserveLeadingUnderscore: false,
 		preserveTrailingDash: false,
+		preserveCharacters: [],
 		...options
 	};
 
@@ -51,11 +69,10 @@ export default function slugify(string, options) {
 		string = decamelize(string);
 	}
 
-	let patternSlug = /[^a-zA-Z\d]+/g;
+	const patternSlug = buildPatternSlug(options);
 
 	if (options.lowercase) {
 		string = string.toLowerCase();
-		patternSlug = /[^a-z\d]+/g;
 	}
 
 	string = string.replace(patternSlug, options.separator);

readme.md

@@ -186,6 +186,24 @@ slugify('foo-bar-', {preserveTrailingDash: true});
 //=> 'foo-bar-'
 ```
 
+##### preserveCharacters
+
+Type: `string[]`\
+Default: `[]`
+
+Preserve certain characters.
+
+It cannot contain the `separator`.
+
+For example, if you want to slugify URLs, but preserve the HTML fragment `#` character.
+
+```js
+import slugify from '@sindresorhus/slugify';
+
+slugify('foo_bar#baz', {preserveCharacters: ['#']});
+//=> 'foo-bar#baz'
+```
+
 ### slugifyWithCounter()
 
 Returns a new instance of `slugify(string, options?)` with a counter to handle multiple occurrences of the same string.

test.js

@@ -186,3 +186,19 @@ test('counter', t => {
 	t.is(slugify2(''), '');
 	t.is(slugify2(''), '');
 });
+
+test('preserve characters', t => {
+	t.is(slugify('foo#bar', {preserveCharacters: []}), 'foo-bar');
+	t.is(slugify('foo.bar', {preserveCharacters: []}), 'foo-bar');
+	t.is(slugify('foo?bar ', {preserveCharacters: ['#']}), 'foo-bar');
+	t.is(slugify('foo#bar', {preserveCharacters: ['#']}), 'foo#bar');
+	t.is(slugify('foo_bar#baz', {preserveCharacters: ['#']}), 'foo-bar#baz');
+	t.is(slugify('foo.bar#baz-quux', {preserveCharacters: ['.', '#']}), 'foo.bar#baz-quux');
+	t.is(slugify('foo.bar#baz-quux', {separator: '.', preserveCharacters: ['-']}), 'foo.bar.baz-quux');
+	t.throws(() => {
+		slugify('foo', {separator: '-', preserveCharacters: ['-']});
+	});
+	t.throws(() => {
+		slugify('foo', {separator: '.', preserveCharacters: ['.']});
+	});
+});